Optical device, optical device manufacturing method, and optical integrated device

ABSTRACT

In a multi-mode interference waveguide (MMI) of a sheet shape spreading in the length direction and the width direction, the length of the multi-mode interference waveguide is set to such a length that the unique mode interferes in the length direction, thereby reducing the coupling loss when inputting/outputting the signal light. The multi-mode interference waveguide has a maximum refraction factor portion in the thickness direction and has such a refraction factor distribution that the refraction factor is reduced as departing from the maximum refraction factor portion. Thus, it is possible to suppress mode dispersion in the thickness direction of the multi-mode interference waveguide and obtain a high transmission rate in the order of 10 Gb/s.

TECHNICAL FIELD

The present invention relates to an optical device having a sheet-formtransmission line used for high-speed multi-mode optical transmissionand a method of manufacturing the optical device, and more specifically,to an optical device using a self-imaging principle of a multi-modeinterference suitable for an optical splitter, an optical combiner, anoptical demultiplexer, an optical multiplexer, an optical straight sheetbus, an optical cross sheet bus, a star coupler, an optical switch andthe like, and a method of manufacturing the optical device. Moreover,the present invention relates to an optical integrated device having theabove-described optical device in a plurality of numbers.

BACKGROUND ART

Research has been performed on an optical device using an opticaltransmission line which optical device is suitable for opticalcommunication systems and the like. It is expected that such an opticaldevice will be applied to an optical data bus sheet for data exchangebetween optical circuits and applied to an optical splitter forsplitting a signal beam and an optical combiner for combining signalbeams. Of the optical transmission lines, a multi-mode opticaltransmission line, which is inexpensive compared to a single-modeoptical transmission line, can take the place of a conventionalelectronic circuit.

An example of the multi-mode optical transmission line is a sheet-formmulti-mode optical transmission line. For example, Document (3)discloses an optical bus circuit board provided with: a sheet-formtransparent medium whose refractive index is homogeneous; a laser diodearray that makes a signal beam incident on an incident end surface ofthe transparent medium; and a photodiode array that receives the signalbeam having exited from the exit side end surface of the transparentmedium. In the optical bus circuit board disclosed in Document (3), theincident beam emitted from the laser diode array is repetitively totallyreflected in the direction of the thickness and in the direction of thewidth inside the transparent medium, exits from the entire area of theexit side end surface as the exiting beam, and is received by thephotodiode array.

Moreover, Document (2) discloses, like Document (3), an optical splitterprovided with: a sheet-form transparent medium whose refractive index ishomogeneous; a laser diode that makes a signal beam incident on thetransparent medium; and a plurality of optical fibers that receives thesignal beam having exited from the transparent medium. In the opticalsplitter described in Document (2), a light diffusing layer is providedon the incident side end surface so that the signal beam is efficientlydiffused inside the transparent medium within a short distance. InDocument (2), the incident beam is also repetitively totally reflectedin the direction of the thickness and in the direction of the widthwithin the transparent medium, exits from the entire area of the exitside end surface as the exiting beam, and is received by the photodiodearray.

Moreover, Document (1) discloses a sheet-form optical data bus having arefractive index distribution such that the highest refractive index isprovided at the center in the direction of the thickness and therefractive index is decreased with distance from the center. In theoptical data bus described in Document (1), the mode dispersion ofmultiple modes is reduced by the refractive index distribution. InDocument (1), the incident beam also exits from the entire area of theexit side end surface as the exiting beam.

On the other hand, there is a technology in which an optical waveguidethat transmits a signal beam in multiple modes in the direction of thewidth is disposed between a single-mode optical transmission line on theincident side and a single-mode optical transmission line on the exitside. This optical waveguide has a predetermined size L in the directionof the length which size L is determined by a uniform refractive index nof the optical waveguide, the basic mode width W₀, in the direction ofthe width, of the optical waveguide and the wavelength λ of thetransmitted signal beam. The optical waveguide generates the exitingbeam by the eigenmodes of the signal beam interfering with each other inthe direction of the length, based on the size L in the direction of thelength (Documents (4) to (8), (11)).

Moreover, in recent years, in the field of optical communications, awavelength division multiplexing (referred to also as WDM) method hasbeen examined in which, in order to increase the communication capacity,a plurality of signals is superimposed on a signal beam of differentwavelengths to be multiplexed and transmitted on the same opticaltransmission line. In the WDM method, optical devices such as an opticaldemultiplexer that demultiplexes signal beams of different wavelengthsand an optical multiplexer that multiplexes signal beams of differentwavelengths play an important role.

A conventional example is known that realizes such an opticaldemultiplexer and an optical splitter by use of the technology in whichan optical waveguide that transmits a signal beam in multiple modes inthe direction of the width is disposed between a single-mode opticaltransmission line on the incident side and a single-mode opticaltransmission line on the exit side (Documents (9), (12) to (15)). Theseconventional optical demultiplexer and optical splitter are connected tothe incident side single-mode waveguide and to the exit side single-modewaveguide, and are provided with the optical waveguide that transmits asignal beam in multiple modes in the direction of the width. In theoptical devices described in Documents (9) and (12) to (15), a multiplexsignal beam of two wavelengths that are different from each other istransmitted in the incident side single-mode waveguide and made incidenton the optical waveguide. The size, in the direction of the width, andthe size, in the direction of the length, of the optical waveguide areset so that the exiting beam is generated in a different position on theexit end by the eigenmodes of the signal beam interfering with eachother in the direction of the length.

Moreover, Document (10) discloses a method of manufacturing an opticaldevice provided with an incident side beam converter, an opticalwaveguide and an exit side beam converter. The optical devicemanufacturing method of Document 10 describes that the optical waveguideis formed by enclosing a fluid material in a glass substrate. Moreover,Document (10) particularly discloses that the incident side beamconverter and the exit side beam converter are provided with arefractive index distribution by successively laminating materialshaving different refractive indices (see Document (10), FIG. 4 and thecorresponding description).

In a case where the multi-mode optical transmission line is made of ahomogeneous medium, when a signal beam is transmitted, the physicaloptical path length (phase velocity) differs among the modes. For thisreason, a phenomenon occurs in which the intensity distribution of theexiting beam varies depending on the length of the optical transmissionline.

Moreover, when the length of the multi-mode optical transmission line islong to an extent that exceeds 100 mm, since the group velocity differsamong the optical paths, a phenomenon occurs in which the signalwaveform of the transmitted beam changes.

As described above, when a mode dispersion occurs which is a reductionin which the phase velocity or the group velocity differs among themodes, the signal beam cannot be transmitted while the intensitydistribution of the incident beam is maintained to the exit side.

To solve the above-mentioned problem, an optical transmission lineprovided with a refractive index distribution is proposed. A signal beampropagating through a medium having a refractive index distributiondraws a curved (meandering) beam locus based on the refractive indexdistribution. By applying this phenomenon, even if the physical opticalpath lengths of the optical paths are different from each other, theoptical path length thereof can be made the same as each other by thedifference in refractive index. Therefore, by appropriately setting therefractive index distribution, a multi-mode optical transmission linecan be obtained since the mode dispersion is suppressed.

For example, Document (1) describes an optical device provided withlaminated sheet-form optical transmission lines and having a refractiveindex distribution in the direction in which the sheet-form opticaltransmission lines are laminated. The sheet-form optical transmissionlines described in Document (1) are capable of transmitting agigabit-class high-frequency signal in multiple modes since the modedispersion is suppressed by the refractive index distribution.

Such an optical device requires a structure for making a signal beamincident on the sheet-form optical transmission lines and making thesignal beam to exit from the sheet-form optical transmission lines. Inthe optical device described in the above-described Document (1), asignal beam is made parallelly incident in the signal beam transmissiondirection from one end of the sheet-form optical transmission lines, andparallelly exits in the signal beam transmission direction from theother end of the sheet-form optical transmission lines (FIGS. 1 and 9 ofDocument (1)).

Moreover, a technology is known in which the optical waveguide(sheet-form optical transmission line) is provided with a mirror forperpendicularly bending the optical axis of the signal beam and theoptical waveguide is coupled to the outside (FIGS. 1 and 2 of Document(16)2). In the optical waveguide described in Document (16), the signalbeam incident from a direction perpendicular to the transmissiondirection is bent by a mirror disposed at 45 degrees from the signalbeam transmission direction and is incident on the optical waveguide.Moreover, the signal beam transmitted through the optical waveguide isbent by a mirror disposed at 45 degrees from the signal beamtransmission direction and exits in a direction perpendicular to thetransmission direction (see FIGS. 1 and 2 of Document (16)).

List of the Documents

-   -   (1) Japanese Laid-Open Patent Publication No. 2000-111738(FIG.        3)    -   (2) Japanese Laid-Open Patent Publication No. 2000-329962(FIG.        2)    -   (3) Japanese Laid-Open Patent Publication No. 2001-147351(FIG.        1)    -   (4) Japanese Laid-Open Patent Publication No. 2003-050330(FIG.        1)    -   (5) Japanese Laid-Open Patent Publication No. 2001-183710(FIG.        1)    -   (6) Japanese Laid-Open Patent Publication No. Hei 1-156703(FIG.        1)    -   (7) U.S. Pat. No. 4,087,159 (FIG. 1)    -   (8) U.S. Pat. No. 4,950,045 (FIG. 1)    -   (9) Japanese Laid-Open Patent Publication No. Hei 8-201648        (pages 2 to 5, FIG. 11)    -   (10) Japanese Laid-Open Patent Publication No.2003-043285 (FIG.        4)    -   (11) Lucas B. Soldano and Eric C. M. Pennings, “Optical        Multi-Mode Interference Device Based on Self-Imaging: Principles        and Applications”, Vol. 13, No.4 Journal of Lightwave        Technology, April, 1995    -   (12) F. Rottmann, A. Neyer, W. Mevenkamp, and E. Voges,        “Integrated-Optic Wavelength Multiplexers on Lithium Niobate        based on Two-Mode Interference”, Journal of Lightwave        Technology” Vol. 6, No. 6 June, 1988    -   (13) M. R. Paiam, C. F. Janz, R. I. MacDonald and J. N.        Broughton, “Compact Planar 980/1550-nm Wavelength        Multi/Demultiplexer Based on Multimode Interference” IEEE        Photonics Technology Letters, Vol. 7, No. 10, October, 1995    -   (14) K. C. Lin and W. Y. Lee, “Guided-wave 1.3/1.55 μm        wavelength division multiplexer based on multimode        interference”, IEEE Electronics Letters, Vol. 32, No. 14, Jul.        4, 1996.    -   (15) Baojun Li, Guozheng Li, Enke Liu, Zuimin Jiang, Jie Qin and        Xun Wang, “Low-Loss 1×2 Multimode Interference Wavelength        Demultiplexer in Silicon-Germanium Alloy” IEEE Photonics        Technology Letters, Vol. 11, No. 5, May, 1999    -   (16) Japanese Laid-Open Patent Publication No. Sho 62-35304(FIG.        1, FIG. 2)

DISCLOSURE OF THE INVENTION

The multi-mode optical transmission lines described in Documents (2) and(3) transmit the incident beam that is incident as a signal beam whiletotally reflecting it in the direction of the thickness and in thedirection of the width inside the transparent medium. For this reason,the optical path length difference occurs among the paths of theincident beam that is incident while being diffused, and this causes themode dispersion. Therefore, in the multi-mode optical transmission linesdescribed in Documents (2) and (3), the transmission speed is limited bythe typical incident beam dispersion, so that transmission at a highspeed exceeding 10 Gbps cannot be performed.

In the optical data bus described in Document (1), since a refractiveindex distribution is provided in the direction of the thickness, themode dispersion in the direction of the thickness is suppressed.However, since the refractive index is uniform in the direction of thewidth, the mode dispersion in the direction of the width occurs, so thatthe transmission speed is limited as well. Moreover, in all of thetechnologies described in Documents (1) to (3), since the incident beamexits from the entire area of the exit side end surface as the exitingbeam, the loss of coupling to the optical transmission line provided onthe exit side is large.

Moreover, in the optical waveguides described in Documents (4) to (8)and (11), when a signal beam that is single-mode in the direction of thethickness is made incident, the coupling loss is small and the signalbeam can be transmitted at high speed. However, even in the case of asingle mode, when an incident beam largely diffused and having a largespread angle or an incident beam with a large beam diameter such as anexiting beam from a multi-mode waveguide is used as the signal beam,since the coupling to the optical waveguide is difficult, the loss ofthe signal beam when the signal beam is incident and exits is large.Moreover, in the optical waveguide described in Documents (4) to (8) and(11), since loss is large with respect to the incident beam that isincident with its axis being shifted from the center of the opticalwaveguide, it is necessary to couple the incident and exiting beams tothe incident and exit sides with high accuracy.

Moreover, when the optical waveguides described in Documents (4) to (8)and (11) are used with a signal beam that is multi-mode in the directionof the thickness being made incident, the problem is improved that it isdifficult to couple the incident and exiting beams with respect to theincident beam largely diffused and having a large spread angle and theincident beam with a large beam diameter. However, when the opticalwaveguides described in Documents (4) to (8) and (11) are used with asignal beam that is multi-mode in the direction of the thickness beingmade incident, the mode dispersion occurs in the direction of thethickness, so that the signal beam cannot be transmitted at high speed.Moreover, in this case, since a plurality of eigenmodes, excited in thedirection of the thickness, of the incident beam that is incident withits axis being shifted in the direction of the thickness interferes inthe direction of the length, the intensity distribution, in thedirection of the thickness, of the exiting beam is changed. When theintensity distribution in the direction of the thickness is changed, theloss when the exiting beam is coupled to the optical transmission lineon the exit side is large.

On the other hand, the optical device manufacturing method described inDocument (10) discloses not an example in which the optical waveguide isprovided with a refractive index distribution but an example in whichthe incident side or exit side beam converter is provided with arefractive index distribution. Therefore, when the process, ofmanufacturing the optical waveguide, of the optical device manufacturingmethod described in Document (10) is used, only the conventional opticaltransmission lines, having a homogeneous refractive index, described inDocuments (1), (2), (4) to (9) and (11) to (15) can be manufactured.Moreover, when the process, of manufacturing the beam converter, of theoptical device manufacturing method described in Document (10) is used,since the size in the direction of the width and the size in thedirection of the length are defined so that a lens function is provided,an optical waveguide that causes a multi-mode interference cannot beobtained. Moreover, the process, of manufacturing the beam converter, ofthe optical device manufacturing method described in Document (10)cannot be said to be a method that is high in productivity when appliedto a method of forming a refractive index distribution in the opticalwaveguide, because it is a method in which materials having differentrefractive indices are successively laminated.

Accordingly, a first object of the present invention is to provide anoptical device having a sheet-form multi-mode optical transmission linewhere the coupling when a signal beam is made incident and made to exitis easy and loss is small, and being capable of high-speed transmissionof approximately 10 Gbs equal to the speed of the signal beamtransmission in a single mode, and a method of manufacturing the opticaldevice. Moreover, the first object of the present invention is toprovide an optical integrated device having the above-described opticaldevices in a plurality of numbers and a method of manufacturing theoptical integrated device.

The above-mentioned first object is achieved by the following firstoptical device:

An optical device that connects, by a signal beam, between an externallyinputted input signal and an output signal to be outputted, is providedwith an optical transmission line being sheet-form and including arefractive index distribution such that a highest refractive index partis provided in a direction of a thickness of the sheet and a refractiveindex does not increase with distance from the highest refractive indexpart in the direction of the thickness, a signal beam corresponding tothe input signal is made incident on the optical transmission line as anincident beam, inside the optical transmission line, the incident beamis transmitted, in a direction of a length that is orthogonal to thedirection of the thickness, in multiple modes having a plurality ofeigenmodes in a direction of a width that is orthogonal to both thedirection of the length and the direction of the thickness, and anexiting beam is generated by the plurality of eigenmodes interferingwith each other in the direction of the length, and the exiting beam ismade to exit from the optical transmission line, and the output signalcorresponding to the exiting beam is outputted.

In the first optical device according to the present invention, sincethe optical transmission line has the refractive index distribution inthe direction of the thickness, even in the case of the structure thattransmits a signal beam in multiple modes, the mode dispersion issuppressed in the direction of the thickness, so that the signal beamcan be transmitted at high speed. Moreover, in the optical deviceaccording to the present invention, since the optical transmission linegenerates the exiting beam by the multi-mode interference, the loss atthe time of incidence and exit is small and a highly accurate adjustmentis unnecessary at the time of connection.

Preferably, the optical transmission line has a size, in the directionof the length, expressed by a function of a difference between apropagation constant of a 0th-order mode excited in the direction of thewidth of the optical transmission line and a propagation constant of aprimary mode. Preferably, the optical transmission line has a size, inthe direction of the length, expressed by a function of a basic modewidth in the direction of the width, an effective refractive index withrespect to a 0-th order mode beam excited in the direction of the widthand a wavelength of a beam transmitted in the multi-mode opticaltransmission line.

Preferably, the optical transmission line includes a refractive indexdistribution such that a central position in the direction of thethickness has the highest refractive index and the refractive index doesnot increase with distance from the central position. In particular, itis preferably that the refractive index distribution changesubstantially along a quadratic function.

Preferably, further, the optical transmission line is made ofpolysilane. In particular, the optical transmission line is made ofpolysilane, and the refractive index distribution is provided by anoxygen concentration distribution when the polysilane is cured.

Preferably, the input signal is an electric signal, and an incidentportion is provided that converts the electric signal into the signalbeam and makes the signal beam incident on the optical transmission lineas the incident beam. As an example, the incident portion has aplurality of light emitting portions disposed in an array in thedirection of the width of the optical transmission line. Moreover,preferably, the input signal is a signal beam, and an incident portionis provided that makes the signal beam incident on the opticaltransmission line as an incident beam.

Preferably, the output signal is an electric signal, and an exit portionis provided that receives the signal beam as an exiting beam havingexited from the optical transmission line and converts the signal beaminto the electric signal. As an example, the exit portion has aplurality of light receiving portions disposed in an array in thedirection of the width of the optical transmission line. Moreover,preferably, the output signal is a signal beam, and an exit portion isprovided that makes the signal beam exit from the optical transmissionline as an exiting beam.

Preferably, the optical device is a 1×N optical splitting device that iscapable of receiving at least one input signal and outputting the inputsignal as a number, N (N=1,2,3, . . . ), of output signals, and theoptical transmission line includes:

-   -   an incident surface for making the incident beam incident; and    -   an exit surface for making the exiting beam exit,    -   the size in the direction of the length is a value that is        substantially an integral multiple of the following expression        when the basic mode width in the direction of the width is W₀,        an effective refractive index with respect to a 0th-order mode        beam excited in the direction of the width is n₀ and the        wavelength of the beam transmitted in the multi-mode optical        transmission line is λ, and    -   one incident beam is made incident on a center in the direction        of the width on the incident surface and a number, N, of exiting        beams are generated symmetrically with respect to the center in        the direction of the width on the exit surface:        $\frac{1}{N} \cdot \frac{n_{0}W_{0}^{2}}{\lambda}$

Preferably, the optical device is an N×1 optical combining device thatis capable of receiving a number, N (N=1,2,3, . . . ), of input signalsand outputting the input signals as at least one output signal, and

-   -   the optical transmission line includes:    -   an incident surface for making the incident beam incident;, and    -   an exit surface for making the exiting beam exit,    -   the size in the direction of the length is a value that is        substantially an integral multiple of the following expression        when the basic mode width in the direction of the width is W₀,        an effective refractive index with respect to a 0th-order mode        beam excited in the direction of the width is n₀ and the        wavelength of the beam transmitted in the multi-mode optical        transmission line is λ, and    -   a number, N, of incident beams all having the same wavelength λ        are made incident symmetrically with respect to a center in the        direction of the width on the incident surface and one exiting        beam is generated at the center in the direction of the width on        the exit surface:        $\frac{1}{N} \cdot \frac{n_{0}W_{0}^{2}}{\lambda}$

Preferably, the optical device is a straight sheet bus that is capableof receiving a number, N (N=1,2,3, . . . ), of input signals andoutputting the input signals as a number, N, of output signalscorresponding one-to-one to the input signals, and

-   -   the optical transmission line includes:    -   an incident surface for making the incident beam incident; and    -   an exit surface for making the exiting beam exit,    -   the size in the direction of the length is a value that is        substantially an integral multiple of the following expression        when the basic mode width in the direction of the width is W₀,        an effective refractive index of a 0th-order mode beam excited        in the direction of the width is n₀ and the wavelength of the        beam transmitted in the multi-mode optical transmission line is        λ, and    -   a number, N, of incident beams all having the same wavelength λ        are made incident on given positions in the direction of the        width on the incident surface and a number, N, of exiting beams        corresponding one-to-one to the number, N, of incident beams are        generated in positions, on the exit surface, whose positions in        the direction of the width are the same as incident positions of        the incident beams: $\frac{8\quad n_{0}W_{0}^{2}}{\lambda}$

Preferably, the optical device is a cross sheet bus that is capable ofreceiving a number, N (N=1,2,3, . . . ), of input signals and outputtingthe input signals as a number, N, of output signals correspondingone-to-one to the input signals, and

-   -   the optical transmission line includes:    -   an incident surface for making the incident beam incident; and    -   an exit surface for making the exiting beam exit,    -   a size in the direction of the length is a value that is        substantially an odd multiple of the following expression when        the basic mode width in the direction of the width is W₀, an        effective refractive index of a 0th-order mode beam excited in        the direction of the width is n₀ and the wavelength of the beam        transmitted in the multi-mode optical transmission line is X,        and a number, N, of incident beams all having the same        wavelength λ are made incident on given positions in the        direction of the width on the incident surface and a number, N,        of exiting beams corresponding one-to-one to the number, N, of        incident beams are generated in positions, on the exit surface,        whose positions in the direction of the width are symmetrical to        incident positions of the incident beams with respect to the        center in the direction of the width:        $\frac{4\quad n_{0}W_{0}^{2}}{\lambda}$

Preferably, the optical device is a star coupler that receives a number,N (N=1,2,3, . . . ), of input signals and outputs the input signals as anumber, N, of output signals corresponding to the input signals, and

-   -   the optical transmission line includes:    -   an incident surface for making the incident beam incident; and    -   an exit surface for making the exiting beam exit,    -   a size in the direction of the length is substantially a value        of the following expression when the basic mode width in the        direction of the width is W₀, an effective refractive index of a        0th-order mode beam excited in the direction of the width is n₀        and the wavelength of the beam transmitted in the multi-mode        optical transmission line is λ, and    -   a number, N, of incident beams all having the same wavelength λ        are made incident on predetermined positions in the direction of        the width on the incident surface and a number, N, of exiting        beams are generated for any one of the incident beams in        positions, on the exit surface, whose positions in the direction        of the width are symmetrical to incident positions of the        incident beams with respect to the center in the direction of        the width:        $\left( {p \pm \frac{1}{N}} \right)\frac{4n_{0}W_{0}^{2}}{\lambda}$        (p is an integer that makes the value inside the parentheses        positive)

Preferably, further, the optical device is a star coupler that receivesa number, N_(EVEN) (N_(EVEN)=2, 4, 6, . . . ), of input signals andoutputs the input signals as a number, N_(EVEN), of output signalscorresponding to the input signals, and the optical transmission linemakes a number, N_(EVEN), of incident beams all having the samewavelength λ incident on positions symmetrical with respect to thecenter in the direction of the width on the incident surface.

Moreover, preferably, further, the optical device is a star coupler thatreceives a number, N_(ODD) (N_(ODD)=1, 3, 5, . . . ), of input signalsand outputs the input signals as a number, N_(ODD), of output signalscorresponding to the input signals, and

-   -   the optical transmission line makes a number, N_(O4141), of        incident beams all having the same wavelength λ incident on        positions asymmetrical with respect to the center in the        direction of the width on the incident surface.

Preferably, the optical device is a two-way straight sheet bus that iscapable of receiving a number, N (N=1,2,3, . . . ), of input signals andoutputting the input signals as a number, N, of output signalscorresponding one-to-one to the first input signals, and is capable ofreceiving a number, M (M=1,2,3, . . . ), of input signals and outputtingthe input signals as a number, M, of output signals correspondingone-to-one to the input signals, and the optical transmission lineincludes:

-   -   a first surface formed at one end in the direction of the        length; and    -   a second surface formed at another end in the direction of the        length,    -   a size in the direction of the length is a value that is        substantially an integral multiple of the following expression        when the basic mode width in the direction of the width is W₀,        an effective refractive index of a 0th-order mode beam excited        in the direction of the width is n₀ and the wavelength of the        beam transmitted in the multi-mode optical transmission line is        X,    -   a number, N, of incident beams all having the same wavelength λ        are made incident on given positions in the direction of the        width on the first surface and a number, N, of exiting beams        corresponding one-to-one to the number, N, of incident beams are        generated in positions, on the second surface, whose positions        in the direction of the width are the same as incident positions        of the incident beams, and    -   a number, M, of incident beams all having the same wavelength λ        as the incident beams on the first surface are made incident on        given positions in the direction of the width on the second        surface and a number, M, of exiting beams corresponding        one-to-one to the number, M, of incident beams are generated in        positions, on the first surface, whose positions in the        direction of the width are the same as incident positions of the        incident beams: $\frac{8\quad n_{0}W_{0}^{2}}{\lambda}$

Preferably, the optical device is a two-way cross sheet bus that iscapable of receiving a number, N (N=1, 2, 3, . . . ), of first inputsignals and outputting the input signals as a number, N, of first outputsignals corresponding one-to-one to the first input signals, and iscapable of receiving a number, M (M=1, 2, 3, . . . ), of second inputsignals and outputting the input signals as a number, M, of outputsignals corresponding one-to-one to the second input signals, and

-   -   the optical transmission line includes:    -   a first surface formed at one end in the direction of the        length; and    -   a second surface formed at another end in the direction of the        length,    -   a size in the direction of the length is a value that is        substantially an odd multiple of the following expression when        the basic mode width in the direction of the width is W₀, an        effective refractive index of a 0th-order mode beam excited in        the direction of the width is n₀ and the wavelength of the beam        transmitted in the multi-mode optical transmission line is λ,    -   a number, N, of incident beams all having the same wavelength λ        are made incident on given positions in the direction of the        width on the first surface and a number, N, of exiting beams        corresponding one-to-one to the number, N, of incident beams are        generated in positions, on the second surface, whose positions        in the direction of the width are symmetrical to incident        positions of the incident beams with respect to the center in        the direction of the width, and    -   a number, M, of incident beams all having the same wavelength λ        are made incident on given positions in the direction of the        width on the second surface and a number, M, of exiting beams        corresponding one-to-one to the number, M, of incident beams are        generated in positions, on the first surface, whose positions in        the direction of the width are symmetrical to incident positions        of the incident beams with respect to the center in the        direction of the width: $\frac{4\quad n_{0}W_{0}^{2}}{\lambda}$

Preferably, the optical transmission line includes: a reflecting surfacethat is formed at one end in the direction of the length and bends anoptical path of the incident beam incident in a direction parallel tothe direction of the thickness, substantially 90 degrees in thedirection of the length; and/or a reflecting surface that is formed atanother end in the direction of the length and bends an optical path ofthe exiting beam transmitted in the direction of the length,substantially 90 degrees so as to exit in a direction parallel to thedirection of the thickness.

Preferably, the optical transmission line includes: a prism that isformed at one end in the direction of the length and bends, in thedirection of the length, an optical path of the incident beam incidentin a direction inclined in the direction of the thickness; and/or aprism that is formed at another end in the direction of the length andbends an optical path of the exiting beam transmitted in the directionof the length, so as to exit in a direction inclined in the direction ofthe thickness.

Preferably, the optical transmission line has a plurality of eigenmodesin the direction of the thickness. According to this structure, anoptical device that uses the multi-mode interference also in thedirection of the thickness can be provided. Preferably, the opticaltransmission line has a thickness of not less than 20 μm.

Preferably, the optical transmission line is curved so that a centralposition in the direction of the thickness always draws the same curveon given two different cross sections including the direction of thelength and the direction of the thickness. Preferably, the opticaltransmission line is twisted so that a central position in the directionof the thickness draws different curves on given two different crosssections including the direction of the length and the direction of thethickness.

The above-mentioned first object is achieved by the following opticalintegrated device:

An optical integrated device that connects, by a signal beam, between anexternally inputted input signal and an output signal to be outputted,is provided with

-   -   a light transmitting portion comprising a plurality of optical        transmission lines being sheet-form and including a refractive        index distribution such that a highest refractive index part is        provided in a direction of a thickness of the sheet and a        refractive index does not increase with distance from the        highest refractive index part in the direction of the thickness,        the optical transmission lines being laminated in the direction        of the thickness,    -   a signal beam corresponding to the input signal is made incident        on the optical transmission lines as an incident beam,    -   inside the optical transmission lines, the incident beam is        transmitted, in a direction of a length that is orthogonal to        the direction of the thickness, in multiple modes having a        plurality of eigenmodes in a direction of a width that is        orthogonal to both the direction of the length and the direction        of the thickness, and an exiting beam is generated by the        plurality of eigenmodes interfering with each other in the        direction of the length, and the exiting beam is made to exit        from the optical transmission lines, and the output signal        corresponding to the exiting beam is outputted.

In the optical integrated device according to the present invention,since the optical transmission lines have the refractive indexdistribution in the direction of the thickness, even in the case of thestructure that transmits a signal beam in multiple modes, the modedispersion is suppressed in the direction of the thickness, so that thesignal beam can be transmitted at high speed. Moreover, in the opticalintegrated device according to the present invention, since the opticaltransmission lines generate the exiting beam by the multi-modeinterference, the loss at the time of incidence and exit is small and ahighly accurate adjustment is unnecessary at the time of connection.

The above-mentioned first object is achieved by the following firstmethod of manufacturing an optical device:

In a method of manufacturing an optical device that connects, by asignal beam, between an externally inputted input signal and an outputsignal to be outputted,

-   -   the optical device is provided with    -   an optical transmission line being sheet-form and including a        refractive index distribution such that a highest refractive        index part is provided in a direction of a thickness of the        sheet and a refractive index does not increase with distance        from the highest refractive index part in the direction of the        thickness,    -   a signal beam corresponding to the input signal is made incident        on the optical transmission line as an incident beam, inside the        optical transmission line, the incident beam is transmitted, in        a direction of a length that is orthogonal to the direction of        the thickness, in multiple modes having a plurality of        eigenmodes in a direction of a width that is orthogonal to both        the direction of the length and the direction of the thickness,        and an exiting beam is generated by the plurality of eigenmodes        interfering with each other in the direction of the length,    -   the exiting beam is made to exit from the optical transmission        line, and the output signal corresponding to the exiting beam is        outputted, and    -   the optical device manufacturing method is provided with:    -   a first step of preparing a forming die that is made of a        material capable of transmitting an energy to be applied to cure        a resin of which the optical transmission line is made, and        includes a concave portion having at least the same depth as the        direction of the thickness of the optical transmission line;    -   a second step of filling the concave portion with the resin;    -   a third step of applying the energy in a predetermined quantity        to the forming die filled with the resin, from above and below        in the direction of the thickness; and    -   a fourth step of, on the resin cured with a desired refractive        index distribution being formed, determining at least a size in        the direction of the length and forming a part of connection of        the incident and exiting beams in order to form the resin into        the optical transmission line.

In the first optical device manufacturing method according to thepresent invention, since the above steps are provided, a sheet-formoptical transmission line including a desired refractive indexdistribution can be easily manufactured with high precision.

Preferably, in the third step,

-   -   the application of the energy is an application of an        ultraviolet ray of a predetermined wavelength, and    -   in the first step,    -   the prepared forming die is made of a material that is        transparent with respect to the ultraviolet ray of the        predetermined wavelength.

Preferably, in the third step, the application of the energy is heating.

Preferably, the optical transmission line includes a refractive indexdistribution such that a central position in the direction of thethickness has the highest refractive index and the refractive index doesnot increase with distance from the central position. Preferably,further, the refractive index distribution changes substantially along aquadratic function.

Preferably, further, the optical transmission line is made ofpolysilane. Moreover, preferably, further, an optical devicemanufacturing method according to claim 35, wherein the opticaltransmission line is made of polysilane, and the refractive indexdistribution is provided by an oxygen concentration distribution whenthe polysilane is cured.

Preferably,

-   -   in the first step, the forming die includes a concave portion        having a size including a plurality of optical transmission        lines to be manufactured, and    -   in the fourth step,    -   a plurality of optical transmission lines is simultaneously        manufactured by cutting the resin.

Preferably,

-   -   in the first step,    -   the forming die includes a concave portion having a size        substantially equal to a size, in the direction of the width, of        the optical transmission line to be manufactured, and    -   in the fourth step,    -   the size in the direction of the length is determined by cutting        the resin.

Preferably,

-   -   in the first step,    -   the forming die includes a concave portion having a size        substantially equal to a size of the optical transmission line        to be manufactured, and    -   in the fourth step,    -   a wall, of the concave portion, situated in a position where the        incident beam and the exiting beam are made incident and made to        exit on and from the optical transmission line is removed.

Preferably,

-   -   a fifth step of releasing the optical transmission line from the        forming die either before or after the fourth step is further        included.

In the optical device manufacturing method of the present invention,since the forming die can be reused by including the fifth step, thecost at the time of manufacture can be reduced.

The above-mentioned second object is achieved by the following secondoptical device:

An optical device that is capable of receiving a multiple signal beamwhere two different wavelengths are superimposed on each other,demultiplexing the multiple signal beam according to the wavelength, andoutputting the multiple signal beam as two different signal beams, isprovided with

-   -   an optical transmission line being sheet-form and including a        refractive index distribution such that a highest refractive        index part is provided in a direction of a thickness of the        sheet and a refractive index does not increase with distance        from the highest refractive index part in the direction of the        thickness,    -   the multiple signal beam is made incident on the optical        transmission line as an incident beam,    -   inside the optical transmission line, the incident beam is        transmitted, in a direction of a length that is orthogonal to        the direction of the thickness, in multiple modes having a        plurality of eigenmodes for each wavelength in a direction of a        width that is orthogonal to both the direction of the length and        the direction of the thickness, and two exiting beams are        generated in different positions in the direction of the width        according to the wavelength by the plurality of eigenmodes        interfering with each other in the direction of the length with        respect to signal beams of the same wavelength, and    -   the two exiting beams are made to exit from the optical        transmission line.

In the second optical device according to the present invention, sincethe optical transmission line has the refractive index distribution inthe direction of the thickness, even in the case of the structure thattransmits a signal beam in multiple modes, the mode dispersion issuppressed in the direction of the thickness, so that the signal beamcanbe transmitted at high speed. Moreover, in the optical device accordingto the present invention, since the optical transmission line generatesthe exiting beam by the multi-mode interference and demultiplexes itaccording to the wavelength, the loss at the time of incidence and exitis small and a highly accurate adjustment is unnecessary at the time ofconnection.

Preferably, the two exiting beams are made to exit from positions in thedirection of the width where a ratio in light quantity between the twoexiting beams is highest. Preferably, the two exiting beams are made toexit from positions in the direction of the width where light quantitiesof the two exiting beams are lowest.

Preferably, the optical transmission line has a size in the direction ofthe length expressed by a function of a difference between a propagationconstant of a 0th-order mode excited in the direction of the width ofthe optical transmission line and a propagation constant of a primarymode.

Preferably, the optical transmission line has a rectangularparallelepiped shape, and has a size in the direction of the lengthexpressed by a function of a basic mode width in the direction of thewidth, the highest refractive index in the direction of the thicknessand a wavelength of a beam transmitted in the multi-mode opticaltransmission line.

Preferably, the optical transmission line includes a refractive indexdistribution such that a central position in the direction of thethickness has the highest refractive index and the refractive index doesnot increase with distance from the central position. In particular, itis preferable that the refractive index distribution changesubstantially along a quadratic function.

The optical devices described in Documents (9) and (12) to (15) have asimilar problem to those of Documents (4) to (8) and (11), because theoptical waveguide performing optical splitting transmits the signal beamin multiple modes only in the direction of the width. That is, even inthe case of a single mode, when a multiplex signal beam largely diffusedand having a large spread angle or a multiplex signal beam with a largebeam diameter is used as the signal beam, since the coupling to theoptical waveguide is difficult, the loss of the signal beam when thesignal beam is incident and exits is large. Moreover, since loss islarge with respect to the incident beam that is incident with its axisbeing shifted from the center of the optical waveguide, it is necessaryto couple the incident multiplex signal beam and the exiting signal beamto the incident and exit sides with high accuracy.

Moreover, when the optical waveguides described in the optical devicesdescribed in Documents (9) and (12) to (15) are used with a multiplexsignal beam that is multi-mode in the direction of the thickness beingincident, the mode dispersion occurs in the direction of the thickness,so that the signal beam cannot be transmitted at high speed. Moreover,in this case, since a plurality of eigenmodes, excited in the directionof the thickness, of the multiplex signal beam that is incident with itsaxis being shifted in the direction of the thickness interferes in thedirection of the length, the intensity distribution in the direction ofthe thickness is changed. When the intensity distribution in thedirection of the thickness is changed, the multiplex signal beam cannotbe demultiplexed or multiplexed.

Accordingly, a second object of the present invention is to provide anoptical device having a sheet-form multi-mode optical transmission linewhere the coupling when a signal beam is made incident and made to exitis easy and loss is small, being capable of high-speed transmission ofapproximately 10 Gbs equal to the speed of the signal beam transmissionin a single mode, and being capable of excellently demultiplexing andmultiplexing a multiplex signal beam.

The above-mentioned second object is achieved by the following thirdoptical device:

An optical device that is capable of receiving two signal beams havingdifferent wavelengths, multiplexing the signal beams and outputting thesignal beams as a multiple signal beam where two different wavelengthsare superimposed on each other, is provided with

-   -   an optical transmission line being sheet-form and including a        refractive index distribution such that a highest refractive        index part is provided in a direction of a thickness of the        sheet and a refractive index does not increase with distance        from the highest refractive index part in the direction of the        thickness,    -   the two signal beams are made incident on the optical        transmission line as incident beams,    -   inside the optical transmission line, the incident beam is        transmitted, in a direction of a length that is orthogonal to        the direction of the thickness, in multiple modes having a        plurality of eigenmodes for each wavelength in a direction of a        width that is orthogonal to both the direction of the length and        the direction of the thickness, and the exiting beam which is a        multiple signal beam is generated in the same position in the        direction of the width according to the wavelength by the        plurality of eigenmodes interfering with each other in the        direction of the length with respect to signal beams of the same        wavelength, and    -   the exiting beam is made to exit from the optical transmission        line.

In the third optical device according to the present invention, sincethe optical transmission line has the refractive index distribution inthe direction of the thickness, even in the case of the structure thattransmits a signal beam in multiple modes, the mode dispersion issuppressed in the direction of the thickness, so that the signal beamcan be transmitted at high speed. Moreover, in the optical deviceaccording to the present invention, since the optical transmission linegenerates the exiting beam by the multi-mode interference andmultiplexes it according to the wavelength, the loss at the time ofincidence and exit is small and a highly accurate adjustment isunnecessary at the time of connection.

None of the optical transmission lines and the optical waveguidesdescribed in Documents (1) to (8) and (11) goes beyond making theexiting beam uniquely exit in correspondence with the incident beam.Therefore, a technical idea of performing switching to select the exitposition of the exiting beam corresponding to the incident beam by useof an optical transmission line or an optical waveguide is notsuggested.

Accordingly, a third object of the present invention is to provide anoptical device having a sheet-form multi-mode optical transmission linewhere the coupling when a signal beam is made incident and made to exitis easy and loss is small, being capable of high-speed transmission ofapproximately 10 Gbs equal to the speed of the signal beam transmissionin a single mode, and being capable of switching of the signal beam tobe transmitted.

The above-mentioned third object is achieved by the following fourthoptical device:

An optical device that connects, by a signal beam, between an externallyinputted input signal and an output signal to be outputted, is providedwith:

-   -   an optical transmission line being sheet-form, including a        refractive index distribution such that a highest refractive        index part is provided in a direction of a thickness of the        sheet and a refractive index does not increase with distance        from the highest refractive index part in the direction of the        thickness, and comprising a first partial optical transmission        line and a second partial optical transmission line adjoining in        a direction of the width orthogonal to the thickness of the        thickness; and    -   refractive index modulating means capable of changing the        refractive index distribution of at least one of the first and        second partial optical transmission lines based on an externally        supplied control signal,    -   selection can be made between a first condition in which the        incident beam is transmitted by use of only the first partial        optical transmission line and a second condition in which the        incident beam is transmitted by use of the first and second        partial optical transmission lines, based on an operation of the        refractive index modulating means,    -   a signal beam corresponding to the input signal is made incident        on the first optical transmission line as the incident beam, in        the first condition,    -   inside the first optical transmission line, the incident beam is        transmitted, in a direction of a length that is orthogonal to        the direction of the thickness and the direction of the width,        in multiple modes having a plurality of eigenmodes in the        direction of the width, the exiting beam is generated by the        plurality of eigenmodes interfering with each other in the        direction of the length, and    -   the exiting beam is made to exit from the first optical        transmission line and the output signal corresponding to the        exiting beam is outputted, and    -   in the second condition,    -   inside the first and second optical transmission lines, the        incident beam is transmitted, in the direction of the thickness,        in multiple modes having a plurality of eigenmodes in the        direction of the width, the exiting beam is generated by the        plurality of eigenmodes interfering with each other in the        direction of the length, and    -   the exiting beam is made to exit from the second optical        transmission line and the output signal corresponding to the        exiting beam is outputted.

In the fourth optical device according to the present invention, sincethe optical transmission line has the refractive index distribution inthe direction of the thickness, even in the case of the structure thattransmits a signal beam in multiple modes, the mode dispersion issuppressed in the direction of the thickness, so that the signal beamcan be transmitted at high speed. Moreover, in the optical deviceaccording to the present invention, since the optical transmission linegenerates the exiting beam by the multi-mode interference and switchesit, the loss at the time of incidence and exit is small and a highlyaccurate adjustment is unnecessary at the time of connection.

Preferably, the refractive index modulating means

-   -   is capable of changing the refractive index distribution of the        first multi-mode partial optical transmission line,    -   in the second condition, makes the refractive index        distributions of the first and second multi-mode partial optical        transmission lines the same as each other, and    -   in the first condition, makes a highest refractive index of the        first multi-mode partial optical transmission line higher than a        highest refractive index of the second multi-mode partial        optical transmission line.

Preferably, an optical device according to claim 49, wherein therefractive index modulating means

-   -   is capable of changing the refractive index distribution of the        second multi-mode partial optical transmission line,    -   in the second condition, makes the refractive index        distributions of the first and second multi-mode partial optical        transmission lines the same as each other, and    -   in the first condition, makes a highest refractive index of the        second multi-mode partial optical transmission line lower than a        highest refractive index of the first multi-mode optical        transmission line.

Preferably, the refractive index modulating means is capable of changingthe refractive index distributions of the first and second multi-modepartial optical transmission lines,

-   -   in the second condition, makes the refractive index        distributions of the first and second multi-mode partial optical        transmission lines the same as each other, and    -   in the first condition, makes a highest refractive index of the        first multi-mode partial optical transmission line higher than a        highest refractive index of the second multi-mode partial        optical transmission line in the second condition, and makes the        highest refractive index of the second multi-mode partial        optical transmission line lower than the highest refractive        index of the first multi-mode partial optical transmission line        in the second condition.

Preferably, of the first and second multi-mode optical transmissionlines, the optical transmission line whose refractive index distributionis changeable by the refractive index modulating means is made of apolymer exhibiting a thermooptic effect, and

-   -   the refractive index modulating means includes a thermal sheet        capable of generating/absorbing heat according to the control        signal, and    -   changes the refractive index distribution by changing a        temperature of the optical transmission line by the thermal        sheet.

Preferably, in the optical transmission line,

-   -   a size in the direction of the length is a value that is        substantially an odd multiple of the following expression when        the basic mode width, in the direction of the width, of the        transmission line is W₀, an effective refractive index of a        0th-order mode beam excited in the direction of the width is n₀        and the wavelength of the beam transmitted in the first and        second optical transmission lines is λ:        $\frac{4\quad n_{0}W_{0}^{2}}{\lambda}$

Preferably, the optical transmission line has a size, in the directionof the width, that is (1/{square root}{square root over ( )}2) timeswith respect to the direction of the width to which the opticaltransmission line is added. According to this structure, the exitingbeam can be generated by the multi-mode interference also in the secondcondition.

Preferably, the optical transmission line includes a refractive indexdistribution such that a central position in the direction of thethickness has the highest refractive index and the refractive index doesnot increase with distance from the central position. In particular, itis preferable that the refractive index distributions changesubstantially along a quadratic function.

In the optical waveguides described in Documents (4) to (8) and (11),the optical waveguide is single-mode both on the input side and on theexit side (the core diameter is 10 μm at most). Therefore, wheneigenmodes that are different in the direction of the width are causedto interfere with each other (multi-mode interference: hereinafter,sometimes referred to as MMI) and the exiting beam is generated based onthe self-imaging principle, it is necessary for the separation betweenthe exiting beams at the output end only to be approximately 10 μmcorresponding to the core diameter of the optical waveguide on theoutput side in the case where one beam is split into two beams. However,when the optical waveguides on the input side and on the output side areboth multi-mode optical waveguides, the core diameter calculated in asimilar manner is as large as approximately 20 μm to 1,000 μm. For thisreason, it is necessary that the separation between the exiting beams atthe output end of the optical waveguide be at least not less than thecore diameter.

The separation between the exiting beams based on the self-imagingprinciple of the multi-mode interference is substantially proportionalto the width of the optical waveguide. Moreover, the size, in thedirection of the length, of the optical waveguide in this case issubstantially proportional to the square of the size in the direction ofthe width. For this reason, when the optical waveguides on the inputside and on the output side are both multi-mode optical waveguides, theconfiguration of the optical waveguides is as large as 2 to 100 times inthe direction of the width and 4 to 10,000 times in the direction of thelength compared to the case of single-mode optical waveguides. Forexample, when 200-μm multi-mode optical fibers are used as the input andoutput waveguides, the size in the direction of the width is 20 timesand the size in the direction of the length is approximately 200 times(specifically, the width: approximately 400 μm, the length:approximately 120,000 μm) compared to the case where single-mode opticalfibers are used; such optical waveguides are large and inferior in thebalance (ratio) between the width and length, and are thereforedifficult to handle.

Further, the profile of the exiting beam generated based on theself-imaging principle of the multi-mode interference is substantiallythe same as the profile of the incident beam. For this reason, when twoor more beams are combined into one beam or one beam is split into twoor more beams, that is, when a larger number of incident and exitingbeams having a large mode field are inputted and outputted at theincident and exit ends, it is necessary that the size, in the directionof the width, of the optical waveguide be increased. As has already beenmentioned, the size, in the direction of the length, of the opticalwaveguide that generates the exiting beam based on the self-imagingprinciple of the multi-mode interference is substantially proportionalto the square of the size in the direction of the width. Therefore, whenthe size, in the direction of the width, of the optical waveguide isincreased, it is necessary that the size in the direction of the lengthbe larger than that in the case where one beam is split into two beams.

Accordingly, a fourth object of the present invention is to provide anoptical device having a sheet-form multi-mode optical transmission linewhere the coupling when a signal beam is made incident and made to exitis easy and loss is small, being capable of high-speed transmission ofapproximately 10 Gbs equal to the speed of the signal beam transmissionin a single mode, and in which the sizes, in the direction of the widthand in the direction of the length, of the sheet-form multi-mode opticaltransmission line are small. Moreover, the fourth object of the presentinvention is to provide a method of manufacturing the above-describedoptical device.

The above-mentioned fourth object is achieved by the following fifthoptical device:

In an optical device for changing a distance between a number, N(N=2,3,4, . . . ), of signal beams disposed on a straight line,

-   -   a number, N, of optical transmission lines are disposed on the        straight line, the optical transmission lines being sheet-form        and including a refractive index distribution such that a        highest refractive index part is provided in a direction of a        thickness of the sheet and a refractive index does not increase        with distance from the highest refractive index part in the        direction of the thickness,    -   the signal beams are made incident on the optical transmission        lines as incident beams,    -   inside the optical transmission lines, the incident beam is        transmitted, in a direction of a length that is orthogonal to        the direction of the thickness, in multiple modes having a        plurality of eigenmodes in a direction of a width that is        orthogonal to both the direction of the length and the direction        of the thickness, and exiting beams are generated in positions        different from positions where the incident beams are incident        on the optical transmission lines in the direction of the width        by the plurality of eigenmodes interfering with each other in        the direction of the length, and    -   the exiting beams are made to exit from the optical transmission        lines as the signal beams.

In the fifth optical device according to the present invention, sincethe above structure is provided, the distance between a plurality ofsignal beams can be easily changed. Consequently, even when a multi-modeoptical fiber or the like is used as the incident and exit portion,connection can be made without any increase in the size of the opticaltransmission line.

Preferably, the optical transmission lines include:

-   -   an incident surface for making the incident beams incident; and    -   an exit surface for making the exiting beams exit, and    -   the incident beams are made incident on given positions in the        direction of the width on the incident surface and the exiting        beams are generated in positions, on the exit surface, whose        positions in the direction of the width are symmetrical to the        incident positions of the incident beams with respect to a        center in the direction of the width.

Preferably, the optical device increases the distance between the signalbeams. Preferably, a sheet-form incident side optical transmission lineis provided, and the optical transmission line is a 1×N opticalsplitting device that splits one incident beam into a number, N, ofbeams and connects the number, N, of exiting beams into which theincident beam is split, to the optical transmission lines as the signalbeams.

The above-mentioned fourth object is achieved by the following sixthoptical device:

An optical device for changing a position of a signal beam is providedwith

-   -   a plurality of optical transmission lines being sheet-form and        including a refractive index distribution such that a highest        refractive index part is provided in a direction of a thickness        of the sheet and a refractive index does not increase with        distance from the highest refractive index part in the direction        of the thickness,    -   the plurality of optical transmission lines are connected in        multiple stages so that an exiting beam having exited from one        of the optical transmission lines becomes an incident beam to be        made incident on another one of the optical transmission lines,    -   the signal beam is made incident on the optical transmission        line as the incident beam,    -   inside the optical transmission lines, the incident beam is        transmitted, in a direction of a length that is orthogonal to        the direction of the thickness, in multiple modes having a        plurality of eigenmodes in a direction of a width that is        orthogonal to both the direction of the length and the direction        of the thickness, and the exiting beam is generated in a        position different from a position where the incident beam is        incident on the optical transmission lines in the direction of        the width by the plurality of eigenmodes interfering with each        other in the direction of the length, and the exiting beam is        made to exit from the optical transmission lines as the signal        beam.

In the sixth optical device according to the present invention, sincethe above structure is provided, the signal beam can be easily shiftedin the direction of the width. Consequently, even when a multi-modeoptical fiber or the like is used as the incident and exit portion,connection can be made without any increase in the size of the opticaltransmission line.

Preferably, the signal beam is a number, N (N=2,3,4, . . . ), of signalbeams disposed on a straight line, a number, N, of optical transmissionlines are disposed on the straight line to change a distance between thenumber, N, of signal beams, the optical transmission lines beingsheet-form and including a refractive index distribution such that ahighest refractive index part is provided in a direction of a thicknessof the sheet and a refractive index does not increase with distance fromthe highest refractive index part in the direction of the thickness,

-   -   the signal beams are made incident on the optical transmission        lines as the incident beams,    -   inside the optical transmission lines, the incident beams are        transmitted, in the direction of the length that is orthogonal        to the direction of the thickness, in multiple modes having a        plurality of eigenmodes in the direction of the width that is        orthogonal to both the direction of the length and the direction        of the thickness, and exiting beams are generated in positions        different from positions where the incident beams are incident        on the optical transmission lines in the direction of the width        by the plurality of eigenmodes interfering with each other in        the direction of the length, and    -   the exiting beams are made to exit from the optical transmission        lines as the signal beam.

The above-mentioned fourth object is achieved by the following seventhoptical device:

An optical device that connects, by a signal beam, between an externallyinputted input signal and an output signal to be outputted, is providedwith:

-   -   a sheet-form optical transmission line being sheet-form and        including a refractive index distribution such that a highest        refractive index part is provided in a direction of a thickness        of the sheet and a refractive index does not increase with        distance from the highest refractive index part in the direction        of the thickness;    -   an incident side optical transmission line that transmits the        incident beam corresponding to the input signal so as to be        incident on the sheet-form optical transmission line;    -   an incident side beam converter that connects the incident side        optical transmission line and the sheet-form optical        transmission line and converts a mode field of the incident side        optical transmission line so that it can be incident on the        sheet-form optical transmission line;    -   an exit side optical transmission line that transmits the        exiting beam from the sheet-form optical transmission line so as        to exit as the output signal; and    -   an exit side beam converter that connects the exit side optical        transmission line and the sheet-form optical transmission line        and converts a mode field of the sheet-form optical transmission        line so that it can be incident on the exit side optical        transmission line,    -   the signal beam exiting from the incident side beam converter is        made incident on the sheet-form optical transmission line as the        incident beam,    -   inside the sheet-form optical transmission line, the incident        beam is transmitted, in a direction of a length that is        orthogonal to the direction of the thickness, in multiple modes        having a plurality of eigenmodes in a direction of a width that        is orthogonal to both the direction of the length and the        direction of the thickness, and the exiting beam is generated by        the plurality of eigenmodes interfering with each other in the        direction of the length, and    -   the exiting beam is made to exit from the sheet-form optical        transmission line and made incident on the exit side beam        converter.

In the seventh optical device according to the present invention, sincethe above structure is provided, the distance between a plurality ofsignal beams can be easily changed. Consequently, even when a multi-modeoptical fiber or the like of a different mode field is used as theincident and exit portion, connection can be made without any increasein the size of the optical transmission line.

Preferably, the incident side beam converter

-   -   is a lens element having a refractive index distribution such        that a highest refractive index is provided at a center and a        refractive index decreases with distance from the center, and    -   is disposed in the same numbers as the signal beams that are        made incident on the sheet-form optical transmission line.

Preferably, the incident side beam converter

-   -   includes the refractive index distribution such that a change in        refractive index between the center and a periphery gradually        increases from a side of the incident side optical transmission        line toward a side of the sheet-form optical transmission line.

Preferably, the incident side beam converter is a waveguide having arefractive index distribution such that the highest refractive index isprovided in a central portion, in a direction parallel to the directionof the thickness, of the sheet-form optical transmission line and therefractive index decreases with distance from the central portion, andis disposed in the same numbers as the signal beams that are madeincident on the sheet-form optical transmission line.

Preferably, the waveguide has a configuration such that a size in thedirection of the width decreases toward a part of connection with thesheet-form optical transmission line. Preferably, further, the incidentside beam converter is formed integrally with the sheet-form opticaltransmission line.

Preferably, the incident side beam converter

-   -   is an optical transmission line having a refractive index        distribution such that a highest refractive index is provided in        a central portion, in a direction parallel to the direction of        the thickness and a direction parallel to the direction of the        width, of the sheet-form optical transmission line and the        refractive index decreases with distance from the central        portion, and    -   the number of incident side beam converters disposed for the        sheet-form optical transmission line is one.

Preferably, the exit side beam converter

-   -   is a lens element having a refractive index distribution such        that a highest refractive index is provided at a center and a        refractive index decreases with distance from the center, and    -   is disposed in the same numbers as the signal beams exiting from        the sheet-form optical transmission line.

Preferably, the exit side optical transmission line

-   -   is an optical fiber having a refractive index distribution such        that a highest refractive index is provided at a center and a        refractive index decreases with distance from the center, and    -   the exit side beam converter    -   includes the refractive index distribution such that a change in        refractive index between the center and a periphery gradually        increases from a side of the exit side optical transmission line        toward a side of the sheet-form optical transmission line.

Preferably, the exit side beam converter

-   -   is a waveguide having a refractive index distribution such that        the highest refractive index is provided in a central portion,        in a direction parallel to the direction of the thickness, of        the sheet-form optical transmission line and the refractive        index decreases with distance from the central portion, and    -   is disposed in the same numbers as the signal beams exiting from        the sheet-form optical transmission line.

Preferably, further, the waveguide has a configuration such that a sizein the direction of the width decreases toward a part of connection withthe sheet-form optical transmission line. Preferably, further, the exitside beam converter is formed integrally with the sheet-form opticaltransmission line.

Preferably, the exit side beam converter

-   -   is an optical transmission line having a refractive index        distribution such that a highest refractive index is provided in        a central portion, in a direction parallel to the direction of        the thickness and a direction parallel to the direction of the        width, of the sheet-form optical transmission line and the        refractive index decreases with distance from the central        portion, and    -   the number of exit side beam converters disposed for the        sheet-form optical transmission line is one.

The above-mentioned object is achieved by the following second opticaldevice manufacturing method:

In a method of manufacturing an optical device that connects, by asignal beam, between an externally inputted input signal and an outputsignal to be outputted,

-   -   the optical device is provided with:    -   a sheet-form optical transmission line being sheet-form and        including a refractive index distribution such that a highest        refractive index part is provided in a direction of a thickness        of the sheet and a refractive index does not increase with        distance from the highest refractive index part in the direction        of the thickness;    -   an incident side optical transmission line that transmits the        incident beam corresponding to the input signal so as to be        incident on the sheet-form optical transmission line;    -   an incident side beam converter that connects the incident side        optical transmission line and the sheet-form optical        transmission line and converts a mode field of the incident side        optical transmission line so that it can be incident on the        sheet-form optical transmission line;    -   an exit side optical transmission line that transmits the        exiting beam from the sheet-form optical transmission line so as        to exit as the output signal; and    -   an exit side beam converter that connects the exit side optical        transmission line and the sheet-form optical transmission line        and converts a mode field of the sheet-form optical transmission        line so that it can be incident on the exit side optical        transmission line, the optical device manufacturing method        comprising: a first step of preparing a forming die that has a        concave portion corresponding to the sheet-form optical        transmission line and at least one of the incident side beam        converter and the exit side beam converter and is made of a        material capable of transmitting an energy to be applied to cure        a resin of which the sheet-form optical transmission line is        made;    -   a second step of filling the concave portion with the resin;    -   a third step of applying the energy in a predetermined quantity        to the forming die filled with the resin, from above and below        in the direction of the thickness to form a desired refractive        index distribution by curing the resin; and    -   a fourth step of, when the incident side beam converter and the        exit side beam converter not formed in the concave portion are        present, connecting the converters to the cured resin, and        further, connecting the incident side optical transmission line        and the exit side optical transmission line.

In the second optical device manufacturing method according to thepresent invention, since the above steps are provided, an optical deviceprovided with a sheet-form optical transmission line and an incidentside optical transmission line including a desired refractive indexdistribution and an exit side optical transmission line can be easilymanufactured with high precision.

Preferably, the application of the energy is an application of anultraviolet ray of a predetermined wavelength, and

-   -   the forming die is made of a material that is transparent with        respect to the ultraviolet ray of the predetermined wavelength.

Moreover, preferably, the application of the energy is heating.

Preferably, a fifth step of releasing the cured resin from the formingdie prior to the fourth step is provided.

Preferably, in the fourth step, when the incident side beam converterand the exit side beam converter not formed in the forming die arepresent, the converters are connected to the cured resin, and further,when the incident side optical transmission line and the exit sideoptical transmission line are connected together, the opticaltransmission lines are disposed on a substrate where a positioningportion for positioning the optical transmission lines is formed.

Preferably, in the first step,

-   -   the forming die includes a positioning portion for positioning        at least one of the incident side optical transmission line and        the exit side optical transmission line, and    -   in the fourth step,    -   the optical transmission lines are disposed on the forming die        where the positioning portion is formed.

Preferably, the incident side optical transmission line is an opticalfiber. Moreover, preferably, the exit side optical transmission line isan optical fiber.

According to the first optical device according to the presentinvention, an optical device can be provided that has a sheet-formmulti-mode optical transmission line where the coupling is easy and lossis small when a signal beam is made incident and made to exit and iscapable of high-speed transmission of approximately 10 Gbs equal to thespeed of the signal beam transmission in a single mode. Moreover,according to the first optical device manufacturing method according tothe present invention, the above-described optical device can bemanufactured.

Moreover, according to the first optical integrated device according tothe present invention, an optical integrated device having theabove-described optical device in a plurality of numbers can beprovided. Moreover, according to the first optical integrated devicemanufacturing method according to the present invention, theabove-described optical integrated device can be manufactured.

According to the second and third optical devices according to thepresent invention, an optical device can be provided that has asheet-form multi-mode optical transmission line where the coupling iseasy and loss is small when a signal beam is made incident and made toexit, is capable of high-speed transmission of approximately 10 Gbsequal to the speed of the signal beam transmission in a single mode andis capable of excellently performing demultiplexing and multiplexing ofa multiple signal beam.

According to the fourth optical device according to the presentinvention, an optical device can be provided that has a sheet-formmulti-mode optical transmission line where the coupling is easy and lossis small when a signal beam is made incident and made to exit, iscapable of high-speed transmission of approximately 10 Gbs equal to thespeed of the signal beam transmission in a single mode, and is capableof switching of the transmitted signal beam.

According to the fifth and sixth optical devices according to thepresent invention, an optical device can be provided that has asheet-form multi-mode optical transmission line where the coupling iseasy and loss is small when a signal beam is made incident and made toexit and is capable of high-speed transmission of approximately 10 Gbsequal to the speed of the signal beam transmission in a single mode andin which the sizes, in the direction of the width and in the directionof the length, of the sheet-form multi-mode optical transmission lineare small.

When the incident and exit direction of the signal beam and the beamtransmission direction of the sheet-form optical transmission linecoincide with each other like the technology described in Document (1),the incident and exit portions and the sheet-form optical transmissionline can be coupled together without any loss. That is, since it is easyto adjust the intensity peak of the signal beam incident on thesheet-form optical transmission line and the refractive indexdistribution of the sheet-form optical transmission line, the loss ofthe signal beam when the signal beam is incident can be made small.

However, in optical devices, it is necessary to dispose an optical partsuch as a laser on the incident side and dispose an optical part such asa sensor on the exit side. For this reason, when these optical parts andthe sheet-form optical transmission line are coupled together, it isnecessary to adjust the height between the optical parts and thesheet-form optical transmission line, so that it is necessary to performpadding when the optical parts are mounted. Consequently, the opticaldevice cannot be made compact.

On the other hand, Document (16) is a technology regarding a single-modesheet-form optical transmission line provided with no refractive indexdistribution. Therefore, in the optical waveguide described in Document(2), the mode dispersion occurs, so that a gigabit-class high-frequencysignal beam cannot be transmitted in multiple modes.

Moreover, in recent years, an optical device that generates a signalbeam by use of the multi-mode interference has been proposed. By usingthe multi-mode interference, an optical splitter that splits theincident signal beam into a plurality of signal beams and an opticalcombiner that combines a plurality of incident signal beams into asingle signal beam can be easily obtained. However, there is nodescription as to the multi-mode interference in either of Document (1)and Document (16).

Accordingly, a fifth object of the present invention is to provide anoptical device where optical parts can be easily mounted and that iscapable of transmitting a gigabit-class high-frequency signal beam inmultiple modes.

Moreover, a sixth object of the present invention is to provide anoptical device where optical parts cab be easily mounted and that iscapable of transmitting a gigabit-class high-frequency signal beam inmultiple modes and is capable of making a signal beam exit by use of themulti-mode interference.

The above-mentioned fifth object is achieved by an eighth optical devicehaving the following structure:

An optical device that transmits an externally incident signal beam andmakes the transmitted signal beam to exit to an outside, is providedwith

-   -   an optical transmission line including a refractive index        distribution in a first direction and being capable of        transmitting the signal beam with a plurality of optical paths        in a second direction orthogonal to the first direction,    -   at least one of an optical axis of the signal beam incident on        the optical transmission line and an optical axis of the signal        beam exiting from the optical transmission line is not parallel        to the second direction, and    -   a phase difference, at the time of incidence on the optical        transmission line, between the two optical paths, of the        plurality of optical paths, incident on the optical transmission        line symmetrically to each other with respect to the optical        axis of the signal beam and a phase difference, at the time of        exit from the optical transmission line, between the two optical        paths are the same.

According to the above structure, since the eighth optical deviceaccording to the present invention is provided with the opticaltransmission line including the refractive index distribution in thefirst direction and being capable of transmitting the signal beam with aplurality of optical paths in the second direction orthogonal to thefirst direction, the mode dispersion is suppressed, so thatgigabit-class high-frequency signal beams can be transmitted in multiplemodes.

Moreover, since at least one of the optical axis of the signal beamincident on the optical transmission line and the optical axis of thesignal beam exiting from the optical transmission line is not parallelto the second direction, it is unnecessary to perform padding when theoptical parts are mounted. Consequently, the overall structure of theoptical device can be made compact.

Further, since the phase difference, at the time of incidence on theoptical transmission line, between the two optical paths, of theplurality of optical paths of the signal beam, incident on the opticaltransmission line symmetrically to each other with respect to theoptical axis of the signal beam and the phase difference, at the time ofexit from the optical transmission line, between the two optical pathsare the same, the intensity distribution of the signal beam at the timeof incidence can be made to exit as the signal beam as it is. That is,since no phase difference is caused by the optical transmission line,the signal beam can be made to exit from the optical transmission linewith the intensity distribution of the incident beam being maintained,so that the signal beam can be made to exit from the opticaltransmission line without any loss.

Preferably, the optical device is provided with the following structure:

An incident portion for making the signal beam incident on the opticaltransmission line, and

-   -   an exit portion for making the signal beam to exit from the        optical transmission line are provided, and    -   at least one of the incident portion and the exit portion is        coupled to the optical transmission line so that the optical        axis of the signal beam transmitted inside is in a direction not        parallel to the second direction.

According to this structure, it is unnecessary to provide the incidentportion or the exit portion at the end surface in the transmissiondirection of the optical transmission line. Consequently, it isunnecessary to perform padding when the optical parts are mounted.

Preferably, at least one of the incident portion and the exit portion iscoupled to the optical transmission line so that the optical axis of thesignal beam transmitted inside is orthogonal to the second direction.

According to this structure, the outside and the optical transmissionline can be easily coupled together. For example, when optical partssuch as a light emitting element that emits the signal beam that isincident on the optical transmission line and a light receiving elementthat receives the signal beam having exited from the opticaltransmission line are coupled to the optical transmission line, theoptical parts can be easily mounted.

Preferably, an optical path length difference between theabove-mentioned two optical axes is equal to an integral multiple of awavelength of the transmitted signal beam (hereinafter, referred to asstructure A). By providing the structure A, the phase difference betweenthe two optical paths can be made zero.

In the structure A, preferably, the two optical axes include a number, m(m=1,2,3, . . . ), of optical path length difference generating portionswhere the optical path length difference is caused, and a sum of theoptical path length differences caused in the number, m, of optical pathlength difference generating portions is equal to a natural multiple ofthe wavelength of the signal beam (hereinafter, referred to as structure1). According to this structure, the phase difference between the twooptical paths can be made zero.

In the structure 1, preferably, the optical transmission line is asheet-form optical transmission line capable of trapping the signal beamin the first direction, and includes a refractive index distributionsuch that a refractive index in a central portion where a thickness inthe first direction is half is the highest and the refractive index doesnot increase with distance from the central portion in the firstdirection. According to this structure, the signal beam is transmittedwhile the mode dispersion is suppressed by the refractive indexdistribution.

In the structure 1, preferably, further, the above-described sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, and the optical path length difference generatingportion is a portion where refractive index histories of the two opticalpaths reflected by the first and second reflecting surfaces aredifferent from each other.

According to this structure, the signal beam incident on the opticaltransmission line from a direction not parallel to the first directioncan be easily made incident on the optical transmission line. Moreover,the signal beam exiting from the optical transmission line in adirection not parallel to the first direction can be easily made to exitfrom the optical transmission line.

In the structure 1, preferably, further, in the above-describedsheet-form optical transmission line, a physical optical path lengthfrom a position where all of the signal beam is bent in the seconddirection by the first reflecting surface to a position immediatelybefore all of the signal beam is incident on the second reflectingsurface is equal to j (j=0,1,2,3, . . . ) times a period of meanderingof an optical path transmitted while meandering based on the refractiveindex distribution. According to this structure, the intensitydistribution of the signal beam is the same between on the incident sideand on the exit side.

In the structure A, preferably, the two optical paths include a number,n (n=2,3,4, . . . ) of optical path length difference generatingportions where an optical path length difference is caused, and

-   -   a sum of the optical path length differences caused in the        number, n, of optical path length difference generating portions        is zero (hereinafter, referred to structure 2). According to        this structure, the phase difference between the two optical        paths can be made zero.

In the structure 2, preferably, the optical transmission line is asheet-form optical transmission line capable of trapping the signal beamin the first direction, and includes a refractive index distributionsuch that a refractive index in a central portion where a thickness inthe first direction is half is the highest and the refractive index doesnot increase with distance from the central portion in the firstdirection. According to this structure, the signal beam is transmittedwhile the mode dispersion is suppressed by the refractive indexdistribution.

In the structure 2, preferably, further, the above-described sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, and the optical path length difference generatingportions are portions where refractive index histories of the twooptical paths reflected by the first and second reflecting surfaces aredifferent from each other.

According to this structure, the signal beam incident on the opticaltransmission line from a direction not parallel to the second directioncan be easily made incident on the optical transmission line. Moreover,the signal beam exiting from the optical transmission line in adirection not parallel to the second direction can be easily made toexit from the optical transmission line.

In the structure 2, preferably, further, in the above-describedsheet-form optical transmission line, a physical optical path lengthfrom a position where all of the signal beam is bent in the seconddirection by the first reflecting surface to a position immediatelybefore all of the signal beam is incident on the second reflectingsurface is equal to (j+0.5) (j=0,1,2,3, . . . ) times a period ofmeandering of an optical path transmitted while meandering based on therefractive index distribution. According to this structure, theintensity distribution of the signal beam is the same between on theincident side and on the exit side.

Preferably, an optical path length difference between the two opticalpaths is zero (hereinafter, referred to as structure B). By providingthe structure B, the phase difference between the two optical paths canbe made zero.

In the structure B, preferably, the two optical paths include a number,n (n=2,3,4, . . . ) of optical path length difference generatingportions where an optical path length difference is caused, and a sum ofthe optical path length differences caused in the number, n, of opticalpath length difference generating portions is zero (hereinafter,referred to as structure 2). According to this structure, the phasedifference between the two optical paths can be made zero.

In the structure 2, preferably, the optical transmission line is asheet-form optical transmission line capable of trapping the signal beamin the first direction, and includes a refractive index distributionsuch that a refractive index in a central portion where a thickness inthe first direction is half is the highest and the refractive index doesnot increase with distance from the central portion in the firstdirection. According to this structure, the signal beam is transmittedwhile the mode dispersion is suppressed by the refractive indexdistribution.

In the structure 2, preferably, further, the above-described sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, and the optical path length difference generatingportions are portions where refractive index histories of the twooptical paths reflected by the first and second reflecting surfaces aredifferent from each other.

According to this structure, the signal beam incident on the opticaltransmission line from a direction not parallel to the second directioncan be easily made incident on the optical transmission line. Moreover,the signal beam exiting from the optical transmission line in adirection not parallel to the second direction can be easily made toexit from the optical transmission line.

In the structure 2, preferably, further, in the above-describedsheet-form optical transmission line, a physical optical path lengthfrom a position where all of the signal beam is bent in the seconddirection by the first reflecting surface to a position immediatelybefore all of the signal beam is incident on the second reflectingsurface is equal to (j+0.5) (j=0,1,2,3, . . . ) times a period ofmeandering of an optical path transmitted while meandering based on therefractive index distribution. According to this structure, theintensity distribution of the signal beam is the same between on theincident side and on the exit side.

In the structure B, preferably, the two optical paths do not have aportion where the optical path length difference is caused (hereinafter,referred to as structure 3). According to this structure, the phasedifference between the two optical paths can be made zero.

In the structure 3, preferably, further, the optical transmission lineis a sheet-form optical transmission line capable of trapping the signalbeam in the first direction, and includes a refractive indexdistribution such that a refractive index in a central portion where athickness in the first direction is half is the highest and therefractive index does not increase with distance from the centralportion in the first direction.

In the structure 3, preferably, further, the sheet-form opticaltransmission line includes: a first reflecting surface for bending anoptical axis of a signal beam incident from a direction not parallel tothe second direction, in the second direction; and a second reflectingsurface for bending an optical axis of a signal beam transmitted in thesecond direction, in the direction not parallel to the second direction,a physical optical path length between the first reflecting surface andthe second reflecting surface in the central portion is equal to j/2(j=0,1,2,3, . . . ) times a period of meandering of an optical pathtransmitted while meandering based on the refractive index distribution,and the signal beam is condensed into a line parallel to a thirddirection orthogonal to both the first direction and the seconddirection in the central portion where the thickness, in the firstdirection, of the optical transmission line is half on the firstreflecting surface and the second reflecting surface.

According to this structure, the first reflecting surface and the secondreflecting surface are optically in a conjugate relationship at thecentral portion. For this reason, the two optical paths do not have apart where an optical path length difference is caused, between thefirst reflecting surface and the second reflecting surface.Consequently, the phase difference between the two optical paths can bemade zero.

Moreover, the above-mentioned sixth object is achieved by a ninthoptical device having the following structure:

An optical device that transmits an externally incident signal beam andmakes the transmitted signal beam exit from a predetermined position toan outside by a multi-mode interference, is provided with:

-   -   a sheet-form optical transmission line including a refractive        index distribution in a first direction, being capable of        transmitting the signal beam in a second direction orthogonal to        the first direction, and being capable of trapping the signal        beam in the first direction;    -   a number, M (M=1,2,3, . . . ), of incident portions for making        the signal beam incident on the sheet-form optical transmission        line; and    -   a number, N (N=1,2,3, . . . ), of exit portions for making the        signal beam exit from the sheet-form optical transmission line,    -   the number, M, of incident portions and the number, N, of exit        portions include at least one nonparallel incident and exit        portion that is coupled to the sheet-form optical transmission        line in a direction where an optical axis of the signal beam        transmitted inside is not parallel to the second direction,    -   between two optical paths incident on the sheet-form optical        transmission line symmetrically to each other with respect to        the optical axis of the signal beam, of a plurality of optical        paths of the signal beam transmitted between the nonparallel        incident and exit portion and the corresponding incident or exit        portion, a phase difference at the time of incidence on the        sheet-form optical transmission line and a phase difference at        the time of exit from the sheet-form optical transmission line        are the same, and the number, M, of incident portions and the        number, N, of exit portions are all disposed in positions        satisfying a predetermined condition of a self-imaging principle        of the multi-mode interference.

According to the above structure, since the ninth optical deviceaccording to the present invention is provided with the opticaltransmission line including the refractive index distribution in thefirst direction and being capable of transmitting the signal beam in thesecond direction orthogonal to the first direction by a plurality ofoptical paths, the mode dispersion is suppressed, so that gigabit-classhigh-frequency signal beams can be transmitted in multiple modes.

Moreover, since the nonparallel incident and exit portion is included,it is unnecessary to perform padding when the optical parts are mounted.Consequently, the overall structure of the optical device can be madecompact.

Moreover, since the phase difference, at the time of incidence on theoptical transmission line, between the two optical paths, of theplurality of optical paths of the signal beam, incident on the opticaltransmission line symmetrically to each other with respect to theoptical axis of the signal beam and the phase difference, at the time ofexit from the optical transmission line, between the two optical pathsare the same, the intensity distribution of the signal beam at the timeof incidence can be made to exit as the signal beam as it is. That is,since no phase difference is caused by the optical transmission line,the signal beam can be made to exit from the optical transmission linewith the intensity distribution of the incident beam being maintained,so that the signal beam can be made to exit from the opticaltransmission line without any loss.

Further, the number, M, of incident portions and the number, N, of exitportions are all disposed in positions satisfying the predeterminedcondition of the self-imaging principle of the multi-mode interference,the signal beam can be controlled by use of the multi-mode interference.Consequently, an optical device such as an optical splitter or anoptical combiner can be obtained.

Preferably, the nonparallel incident portion is coupled to the opticaltransmission line so that the optical axis of the signal beamtransmitted inside is orthogonal to the second direction. According tothis structure, the outside and the optical transmission line can beeasily coupled together. For example, when optical parts such as a lightemitting element that emits the signal beam that is incident on theoptical transmission line and a light receiving element that receivesthe signal beam having exited from the optical transmission line arecoupled to the optical transmission line, the optical parts can beeasily mounted.

Preferably, an optical path length difference between the two opticalpaths is equal to an integral multiple of a wavelength of thetransmitted signal beam (hereinafter, referred to structure A). Byproviding the structure A, the phase difference between the two opticalpaths can be made zero.

In the structure A, preferably, the two optical paths include a number,m (m=1,2,3, . . . ) of optical path length difference generatingportions where the optical path length difference is caused, and a sumof the optical path length differences caused in the number, m, ofoptical path length difference generating portions is equal to a naturalmultiple of the wavelength of the signal beam (hereinafter, referred toas structure 1). According to this structure, the phase differencebetween the two optical paths can be made zero.

In the structure 1, preferably, the optical transmission line includes arefractive index distribution such that a refractive index in a centralportion where a thickness in the first direction is half is the highestand the refractive index does not increase with distance from thecentral portion in the first direction. According to this structure, thesignal beam is transmitted while the mode dispersion is suppressed bythe refractive index distribution.

In the structure 1, preferably, further, the above-described sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, the optical path length difference generatingportion is a portion where refractive index histories of the two opticalpaths reflected by the first and second reflecting surfaces aredifferent from each other.

According to this structure, the signal beam incident on the opticaltransmission line from a direction not parallel to the first directioncan be easily made incident on the optical transmission line. Moreover,the signal beam exiting from the optical transmission line in adirection not parallel to the first direction can be easily made to exitfrom the optical transmission line.

In the structure 1, preferably, further, in the above-describedsheet-form optical transmission line, a physical optical path lengthfrom a position where all of the signal beam is bent in the seconddirection by the first reflecting surface to a position immediatelybefore all of the signal beam is incident on the second reflectingsurface is equal to j (j=0,1,2,3, . . . ) times a period of meanderingof an optical path transmitted while meandering based on the refractiveindex distribution. According to this structure, the intensitydistribution of the signal beam is the same between on the incident sideand on the exit side.

In the structure A, preferably, the two optical paths include a number,n (n=2,3,4, . . . ) of optical path length difference generatingportions where the optical path length difference is caused, and a sumof the optical path length differences caused in the number, n, ofoptical path length difference generating portions is zero (hereinafter,referred to as structure 2). According to this structure, the phasedifference between the two optical paths can be made zero.

In the structure 2, preferably, the optical transmission line includes arefractive index distribution such that a refractive index in a centralportion where a thickness in the first direction is half is the highestand the refractive index does not increase with distance from thecentral portion in the first direction. According to this structure, thesignal beam is transmitted while the mode dispersion is suppressed bythe refractive index distribution.

In the structure 2, preferably, further, the above-described sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, and the optical path length difference generatingportions are portions where refractive index histories of the twooptical paths reflected by the first and second reflecting surfaces aredifferent from each other.

According to this structure, the signal beam incident on the opticaltransmission line from a direction not parallel to the second directioncan be easily made incident on the optical transmission line. Moreover,the signal beam exiting from the optical transmission line in adirection not parallel to the first direction can be easily made to exitfrom the optical transmission line.

In the structure 2, preferably, further, in the sheet-form opticaltransmission line, a physical optical path length from a position whereall of the signal beam is bent in the second direction by the firstreflecting surface to a position immediately before all of the signalbeam is incident on the second reflecting surface is equal to (j+0.5)(j=0,1,2,3, . . . ) times a period of meandering of an optical pathtransmitted while meandering based on the refractive index distribution.According to this structure, the intensity distribution of the signalbeam is the same between on the incident side and on the exit side.

Preferably, an optical path length difference between the two opticalpaths is zero (hereinafter, referred to as structure B). By providingthe structure B, the phase difference between the two optical paths canbe made zero.

In the structure B, preferably, the two optical paths include a number,n (n=2,3,4, . . . ) of optical path length difference generatingportions where the optical path length difference is caused, and a sumof the optical path length differences caused in the number, n, ofoptical path length difference generating portions is zero (hereinafter,referred to as structure 2). According to this structure, the phasedifference between the two optical paths can be made zero.

In the structure 2, preferably, the optical transmission line includes arefractive index distribution such that a refractive index in a centralportion where a thickness in the first direction is half is the highestand the refractive index does not increase with distance from thecentral portion in the first direction. According to this structure, thesignal beam is transmitted while the mode dispersion is suppressed bythe refractive index distribution.

In the structure 2, preferably, further, the above-described sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, and the optical path length difference generatingportions are portions where refractive index histories of the twooptical paths reflected by the first and second reflecting surfaces aredifferent from each other.

According to this structure, the signal beam incident on the opticaltransmission line from a direction not parallel to the second directioncan be easily made incident on the optical transmission line. Moreover,the signal beam exiting from the optical transmission line in adirection not parallel to the second direction can be easily made toexit from the optical transmission line.

In the structure 2, preferably, further, in the above-describedsheet-form optical transmission line, a physical optical path lengthfrom a position where all of the signal beam is bent in the seconddirection by the first reflecting surface to a position immediatelybefore all of the signal beam is incident on the second reflectingsurface is equal to (j+0.5) (j=0,1,2,3, . . . ) times a period ofmeandering of an optical path transmitted while meandering based on therefractive index distribution. According to this structure, theintensity distribution of the signal beam is the same between on theincident side and on the exit side.

In the structure B, preferably, the two optical paths do not have aportion where the optical path length difference is caused (hereinafter,referred to as structure 3). According to this structure, the phasedifference between the two optical paths can be made zero.

In the structure 3, preferably, the optical transmission line includes arefractive index distribution such that a refractive index in a centralportion where a thickness in the first direction is half is the highestand the refractive index does not increase with distance from thecentral portion in the first direction.

In the structure 3, preferably, further, the above-described sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, a physical optical path length between the firstreflecting surface and the second reflecting surface in the centralportion is equal to j/2 (j=0,1,2,3, . . . ) times a period of meanderingof an optical path transmitted while meandering based on the refractiveindex distribution, and the signal beam is condensed into a lineparallel to a third direction orthogonal to both the first direction andthe second direction in the central portion where the thickness, in thefirst direction, of the optical transmission line is half on the firstreflecting surface and the second reflecting surface.

According to this structure, the first reflecting surface and the secondreflecting surface are optically in a conjugate relationship at thecentral portion. For this reason, the two optical paths do not have apart where an optical path length difference is caused, between thefirst reflecting surface and the second reflecting surface.Consequently, the phase difference between the two optical paths can bemade zero.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view showing the general outline of a gradedindex slab waveguide of an optical device that splits one beam into twobeams according to a first embodiment of the present invention.

FIG. 1B is a cross-sectional view of the graded index slab waveguide ofthe optical device that splits one beam into two beams according to thefirst embodiment of the present invention.

FIG. 2 is a perspective view showing the general outline of a gradedindex slab waveguide of an optical device that splits one beam intoeight beams according to a second embodiment of the present invention.

FIG. 3 is a perspective view showing the general outline of a two-signalstraight sheet bus which is an optical device according to a thirdembodiment of the present invention.

FIG. 4 is a perspective view showing the general outline of aneight-signal straight sheet bus which is an optical device according toa modification of the third embodiment of the present invention.

FIG. 5 is a perspective view showing the general outline of a two-signalcross sheet bus which is an optical device according to a fourthembodiment of the present invention.

FIG. 6 is a perspective view showing the general outline of aneight-signal cross sheet bus which is an optical device according to amodification of the fourth embodiment of the present invention.

FIG. 7 is a perspective view showing the general outline of a two-signalstar coupler which is an optical device according to a fifth embodimentof the present invention.

FIG. 8A is a perspective view showing the general outline of a one sidecontrol type optical switch which is an optical device according to asixth embodiment of the present invention.

FIG. 8B is a perspective view showing the general outline of a both sidecontrol type optical switch which is an optical device according to afirst modification of the sixth embodiment of the present invention.

FIG. 9 is a perspective view showing the general outline of an opticalswitch array which is an optical integrated device according to aseventh embodiment of the present invention.

FIG. 10 is a perspective view showing the general outline of asingle-pair two-way straight sheet bus which is an optical deviceaccording to an eighth embodiment of the present invention.

FIG. 11 is a perspective view showing the general outline of a four-pairtwo-way straight sheet bus which is an optical device according to amodification of the eighth embodiment of the present invention.

FIG. 12 is a perspective view showing the general outline of asingle-pair two-way cross sheet bus which is an optical device of aninth embodiment of the present invention.

FIG. 13 is a schematic diagram of the structure of a single-pair two-waystraight sheet bus array which is an optical integrated device accordingto a tenth embodiment of the present invention.

FIG. 14 is a schematic diagram of the structure of a multi-layer opticalbus which is an optical integrated device according to an eleventhembodiment of the present invention.

FIG. 15A is a perspective view showing an example of the incidence andexit method of the graded index slab waveguide.

FIG. 15B is a perspective view showing another example of the incidenceand exit method of the graded index slab waveguide.

FIG. 16 is a perspective view showing still another example of theincident and exit method of the graded index slab waveguide.

FIG. 17A is a perspective view showing an example of the configurationof the graded index slab waveguide.

FIG. 17B is a perspective view showing another example of theconfiguration of the graded index slab waveguide.

FIG. 18 is a result of a BPM (beam propagation method) simulation in thecase where one signal beam is split into two beams.

FIG. 19 is a top view showing a graded index slab waveguide of a starcoupler having three input and output beams according to the presentinvention.

FIG. 20A is a perspective view showing the general outline of a one sidecontrol type optical switch which is an optical device according to asecond modification of the sixth embodiment of the present invention.

FIG. 20B is a perspective view showing the general outline of a bothside control type optical switch which is an optical device according toa third modification of the sixth embodiment of the present invention.

FIG. 21A is a perspective view showing the general outline of a gradedindex slab waveguide of an optical device that performs beammultiplexing according to a twelfth embodiment of the present invention.

FIG. 21B is a cross-sectional view of the graded index slab waveguide ofthe optical device that performs beam multiplexing according to thetwelfth embodiment of the present invention.

FIG. 22A is a result of a BPM simulation performed when a signal beam of1.30 μm is transmitted through the graded index slab waveguide.

FIG. 22B is a result of a BPM simulation performed when a signal beam of1.55 μm is transmitted through the graded index slab waveguide 1201.

FIG. 23 is a schematic diagram of the structure of an optical devicewhich is a 1×2 optical splitter according to a thirteenth embodiment ofthe present invention.

FIG. 24 is a top view showing a relevant part of an optical device forincreasing the distance among three or more signal beams according to afirst modification of the thirteenth embodiment of the presentinvention.

FIG. 25 is a top view showing a relevant part of an optical device forincreasing the distance between signal beams according to a secondmodification of the thirteenth embodiment of the present invention.

FIG. 26 is a perspective view showing the general outline of thestructure of an optical device having a beam converter according to afourteenth embodiment of the present invention.

FIG. 27 is a perspective view showing the general outline of thestructure of an optical device according to a first modification of thefourteenth embodiment of the present invention.

FIG. 28A is a top view showing the general outline of the structure ofan optical device according to a second modification of the fourteenthembodiment of the present invention.

FIG. 28B is a cross-sectional view showing an example of an exit sidebeam converter of the optical device according to the secondmodification of the fourteenth embodiment of the present invention.

FIG. 28C is a cross-sectional view showing another example of the exitside beam converter of the optical device according to the secondmodification of the fourteenth embodiment of the present invention.

FIG. 29 is a perspective view showing the general outline of thestructure of an optical device according to a third modification of thefourteenth embodiment of the present invention.

FIGS. 30 are explanatory views showing another example of the method ofmanufacturing the graded index slab waveguide.

FIGS. 31 are explanatory views showing another example of the method ofmanufacturing the graded index slab waveguide.

FIGS. 32 are explanatory views explaining the mechanism of therefractive index distribution using polysilane.

FIG. 33 is an explanatory view explaining a method of manufacturing theoptical device according to the first modification of the fourteenthembodiment of the present invention.

FIG. 34 is an explanatory view explaining another example of the methodof manufacturing the optical device according to the first modificationof the fourteenth embodiment of the present invention.

FIG. 35 is an explanatory view explaining another example of the methodof manufacturing the optical device according to the first modificationof the fourteenth embodiment of the present invention.

FIG. 36A is a perspective view of a multi-mode interference 1×2 splitteraccording to a fifteenth embodiment of the present invention.

FIG. 36B is a front view of the multi-mode interference 1×2 splitter.

FIG. 37 is a cross-sectional view of a part, where the signal beam istransmitted, of the multi-mode interference 1×2 splitter according tothe fifteenth embodiment of the present invention.

FIG. 38A is a cross section, taken on a plane including the C-D-G-Hplane in FIG. 36A, of a sheet-form optical transmission line and anincident portion.

FIG. 38B is a graph showing the refractive index distribution of thesheet-form optical transmission line.

FIG. 39 is a cross-sectional view of a part, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter according to asixteenth embodiment of the present invention.

FIG. 40 is a cross-sectional view of a part, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter according to aseventeenth embodiment of the present invention.

FIG. 41A is a cross-sectional view of a part, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter according to aneighteenth embodiment of the present invention.

FIG. 41B is a cross-sectional view of apart, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter according to anineteenth embodiment of the present invention.

FIG. 42A is a cross-sectional view of a part, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter according to atwentieth embodiment of the present invention.

FIG. 42B is a cross-sectional view of a part, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter according to atwenty-first embodiment of the present invention.

FIG. 43A is a cross-sectional view of apart, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter according to atwenty-second embodiment of the present invention.

FIG. 43B is across-sectional view of apart, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter according to atwenty-third embodiment of the present invention.

FIG. 44 is a perspective view showing the structure of the multi-modeinterference 1×2 splitter according to the fifteenth embodiment of thepresent invention.

FIG. 45 is a partial cross-sectional view of the sheet-form opticaltransmission line according to the fifteenth embodiment.

FIGS. 46A to 46D are schematic views each showing an example of theinput and output structure of the optical device.

FIG. 47A is a perspective view showing the general outline of a gradedindex slab waveguide of an optical device that splits one beam into twobeams according to a twenty-fourth embodiment of the present invention.

FIG. 47B is a perspective view showing the general outline of a gradedindex slab waveguide of an optical device that splits one beam into twobeams according to a modification of the twenty-fourth embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. The present invention is not limited to theembodiments described below. Moreover, the present invention includescombinations of the embodiments described below. In the graded indexslab waveguides according to the embodiments, the direction parallel tothe z-axis is defined as the direction of the length, the directionparallel to the y-axis is defined as the direction of the length and thedirection parallel to the x-axis is defined as the direction of thewidth. In particular, the direction, toward the positive side, of thez-axis is defined as the transmission direction. Moreover, in the gradedindex slab waveguides according to the embodiments, the size in thedirection parallel to the x-axis is the slab width (W), the size in thedirection parallel to the y-axis is the slab thickness (D), and the sizein the direction parallel to the z-axis is the slab length L. Unlessotherwise specified, in the figures, the refractive index distributionis schematically shown by the shading in the figures, and the darker theshading is, the higher the refractive index is.

First Embodiment

FIG. 1A is a perspective view showing the general outline of a gradedindex slab waveguide 101 of an optical device that splits one beam intotwo beams according to a first embodiment of the present invention. FIG.1B is a cross-sectional view of the graded index slab waveguide 101 ofthe optical device that splits one beam into two beams according to thefirst embodiment of the present invention. FIG. 18 is a result of a BPM(beam propagation method) simulation in the case where one beam is splitinto two beams.

The optical device according to the first embodiment comprises as a mainelement the graded index slab waveguide 101 that transmits beams. Thegraded index slab waveguide 101 is, as shown in FIG. 1A, a sheet-formmulti-mode optical transmission line that extends parallel to the x-zplane. The graded index slab waveguide 101 has a refractive indexdistribution in the direction of the thickness such that the highestrefractive index n_(max) is provided at the central position in thedirection of the thickness and the refractive index does not increasewith distance from the center. The graded index slab waveguide 101 has auniform refractive index in the direction of the width and has norefractive index distribution. The graded index slab waveguide 101includes an incident surface 102 and an exit surface 103. The incidentsurface 102 is opposed to an incident portion (not shown) that makes anincident beam 107 incident on the central position in the direction ofthe width. The exit surface 103 is opposed to an exit portion (notshown) that receives two exiting beams 108 that exit from positionssymmetrical with respect to the center in the direction of the width.The incident portion makes the incident beam 107 corresponding to asignal beam incident on the central position, in the direction of thewidth, of the incident surface 102. The incident beam 107 is transmittedinside the graded index slab waveguide 101. The incident beam 107 issplit into two beams according to the self-imaging principle of themulti-mode interference described later inside the graded index slabwaveguide 101, and exits as the two exiting beams 108 from positions,apart from each other in the direction of the width, of the exit surface103 to reach the exit portion.

The slab length L of the graded index slab waveguide 101 issubstantially n₀×W₀ ²/(2λ), and the distance D1, in the direction of thewidth, between the exit positions of the two exiting beams 108 issubstantially, W₀/2. Here, W₀ is the width of the basic mode, in thedirection of the width, of the graded index slab waveguide 101, and n₀is the effective refractive index of the 0th-order mode beam excited inthe direction of the width. The effective refractive index n₀ is aconstant determined by the highest refractive index n_(max) in thedirection of the thickness and the configuration of the graded indexslab waveguide 101. However, behind the position where the slab length Lcoincides with n₀×W₀ ²/(2λ), similar output is repeated every lengthn₀×W₀ ²/λ by the self-imaging principle. Therefore, by the slab length Lbeing an odd multiple of n₀×W₀ ²/(2λ), the slab length L can be adjustedso as to be a desired length.

For example, a case will be considered where in the graded index slabwaveguide 101 shown in FIG. 1A, the effective refractive index n₀ withrespect to the 0-th order mode beam excited in the direction of thewidth is approximately 1.5, the wavelength λ of the transmitted beam is1.30 μm, the slab width W of the graded index slab waveguide 101 is 400μm and the slab thickness D of the graded index slab waveguide 101 is 50μm. The width W₀ of the basic mode, in the direction of the width, ofthe graded index slab waveguide 101 is dependent on the refractive indexn₁ of the surrounding of the graded index slab waveguide 101. When thesurrounding of the graded index slab waveguide 101 is air (n₁=1), sincethe value of W₀ is 400.16 μm, the value of the shortest slab length L ofthe graded index slab waveguide 101 is approximately 92,400 μm. At thistime, the value of the distance D1 between the exit positions of theexiting beams 108 is 200.08 μm. FIG. 18 shows the result of the BPMsimulation performed under the above-described conditions. In FIG. 18, amanner is seen in which the incident beam 107 incident on the centralposition, in the direction of the width, of the incident surface 102 issplit into five beams, four beams and then, three beams whileinterfering in multiple modes in the direction of the length, and in theend, the incident beam 107 is split into two beams at the exit surfaceas designed.

The refractive index distribution, in the direction of the thickness, ofthe graded index slab waveguide 101 is approximately expressed, forexample, by the highest refractive index n_(max) at the central positionin the direction of the thickness situated at the center in thedirection of the thickness, the distance r away from the centralposition in the direction of the thickness and a refractive indexdistribution constant A^(1/2) as shown by (Expression 1):$\begin{matrix}{n = {n_{\max}\left( {1 - {A\frac{r^{2}}{2}}} \right)}} & \left( {{Expression}\quad 1} \right)\end{matrix}$

It is difficult for the actual refractive index distribution tocompletely coincide with (Expression 1) because of the difficulty ofcontrolling the manufacturing process. The graded index slab waveguide101 according to the first embodiment has a structure in which the partof the highest refractive index is formed in the vicinity of the centerand the refractive index decreases according to the parabola as definedby (Expression 1) according to the distance from the part of the highestrefractive index.

In actuality, in the signal beam transmitted through the graded indexslab waveguide 101, a plurality of modes is excited in the direction ofthe width and the effective refractive index differs among the modes. Asmentioned later, in the MMI, since the size in the direction of thelength is a function of the effective refractive index of the 0th-ordermode beam excited in the direction of the width, it is more convenientto replace the highest refractive index n_(max) with the effectiverefractive index n₀ of the 0th-order mode beam excited in the directionof the width. Therefore, in the following discussion, the effectiverefractive index n₀ of the 0th-order mode beam excited in the directionof the width is used as the refractive index. The effective refractiveindex n₀ is determined by the highest refractive index n_(max), thewavelength of the signal beam and the configuration of the sheet-formoptical transmission line.

The refractive index distribution constant is optimized according to thefilm thickness of the graded index slab waveguide 101 and the profile ofthe incident beam 107 so that the beam transmitted in the graded indexslab waveguide 101 does not spread outside the film thickness. Forexample, when the spread angle of the incident beam 107 is largecompared to the film thickness of the graded index slab waveguide 101,the refractive index distribution constant is increased. Conversely,when the spread angle of the incident beam 107 is low, the refractiveindex distribution constant is decreased. Moreover, by adjusting thefilm thickness of the graded index slab waveguide 101 in accordance withthe beam diameter of the incident beam 107, the coupling loss can bereduced. The refractive index distribution is not necessarily acontinuous change as shown in (Expression 1); it may step wisely changeas a function of the distance from the center.

Next, a mechanism will be described of, when an incident beam incidentsymmetrically with respect to the central line in the direction of thewidth is incident on the incident surface 102 of the graded index slabwaveguide 101, splitting the incident beam into two beams symmetricallywith respect to the central line in the direction of the width on theside of the exit surface 103. The case (i) of a beam transmitted withinthe central plane in the direction of the thickness (signal beamtransmitted on the optical path designated A in FIG. 1B and (ii) a beamnot transmitted within the central plane in the direction of thethickness will be described separately. As the beam of (ii) nottransmitted within the central plane in the direction of the thickness,the following two signal beams are present: the case of an incident beamthat is incident with an angle of axis shift on the central plane(signal beam transmitted on the optical path designated B in FIG. 1B)and the case of an incident beam that is incident on a positionposition-shifted (axis-shifted) from the central plane (signal beamtransmitted on the optical path designated C in FIG. 1B). The beam of(i) transmitted within the central plane in the direction of thethickness is not affected by the refractive index distribution in thedirection of the thickness. On the other hand, the beam of (ii) nottransmitted within the central plane in the direction of the thicknessis affected by the refractive index distribution in the direction of thethickness.

In the graded index slab waveguide 101, the behavior of the beam of (i)transmitted within the central plane in the direction of the thicknessis equivalent to that in a case where a uniform refractive index is theeffective refractive index n₀ with respect to the 0th-order mode excitedin the direction of the width in the slab waveguide described inDocument (11), because it is affected substantially only by theeffective refractive index n₀. Therefore, the condition of the exitingbeams with respect to the incident beam transmitted within the centralplane, in the direction of the thickness, of the graded index slabwaveguide 101 varies according to the slab length L by the multi-modemode dispersion excited in the direction of the width of the slabwaveguide whose refractive index is n₀ and uniform. Here, that thecondition of the exiting beams varies means that the number and exitpositions of images the same as the incident beam vary. The graded indexslab waveguide 101 according to the first embodiment is structured sothat by the slab length L substantially coinciding with n₀×W₀ ²/λ/2 andmaking one incident beam 107 incident on the central position, in thedirection of the width, of the incident surface 102, images the same asthe incident beam 107 are formed on the exit surface 103 at an intervalof substantially W₀/2 so as to be symmetrical with respect to the centerin the direction of the width. The graded index slab waveguide 101outputs two images formed on the exiting surface 103 as the two exitingbeams 108. The two exiting beams 108 have the same profile since theyare outputs of the same image as the incident beam 107.

By using this self-imaging principle of the multi-mode interference(MMI), a device having functions shown in the following (1) and (2) canbe formed in accordance with the incident position in the direction ofthe width:

(1) Asymmetrical Incidence

In a slab waveguide having the uniform refractive index n₀, with respectto an incident beam shifted by x from the center in the direction of thewidth, an exiting beam having the same profile as the incident beamexits from the exit surface with the position and number thereof beingvaried according to the slab length L as shown by the following(Expression 2) to (Expression 8). Here, p and N are integers. Moreover,the integer p is an integer where (p±1/N) is positive. (1-1)$\begin{matrix}{L = {p\frac{8n_{0}W_{0}^{2}}{\lambda}}} & \left( {{Expression}\quad 2} \right)\end{matrix}$

By the slab length L satisfying (Expression 2), an exiting beam can bemade to exit from a position corresponding to the incident beam in thedirection of the width of the exit surface, that is, a position shiftedby x, from the center in the direction of the width, in the samedirection as that in the case of the incident beam. (1-2)$\begin{matrix}{L = {\left( {{2p} + 1} \right)\frac{4n_{0}W_{0}^{2}}{\lambda}}} & \left( {{Expression}\quad 3} \right)\end{matrix}$

By the slab length L satisfying (Expression 3), an exiting beam can bemade to exit from a position symmetrical to the incident beam withrespect to the center in the direction of the width in the direction ofthe width of the exit surface, that is, a position shifted by x, fromthe center in the direction of the width, in the direction opposite tothat in the case of the incident beam. (1-3) $\begin{matrix}{L = {\left( {p \pm \frac{1}{N}} \right)\frac{4n_{0}W_{0}^{2}}{\lambda}}} & \left( {{Expression}\quad 4} \right)\end{matrix}$

By the slab length L satisfying (Expression 4), a number, N, of exitingbeams can be made to exit from a number, N, of positions between aposition corresponding to the incident beam in the direction of thewidth of the exit surface, that is, a position shifted by x, from thecenter in the direction of the width, in the same direction as that inthe case of the incident beam and a position symmetrical to the incidentbeam with respect to the center in the direction of the width in thedirection of the width of the exit surface, that is, a position shiftedby x, from the center in the direction of the width, in the directionopposite to that in the case of the incident beam.

When the number of incident beams is two and the incident positionsthereof are both shifted by approximately ±W₀/6 from the center, theslab length L is ⅓ the slab length L that is mentioned in (1-1) to (1-3)as shown below. (1-1)^(′) $\begin{matrix}{L = {p\frac{8n_{0}W_{0}^{2}}{3\lambda}}} & \left( {{Expression}\quad 5} \right) \\\left( {1\text{-}2} \right)^{\prime} & \quad \\{L = {\left( {{2p} + 1} \right)\frac{4n_{0}W_{0}^{2}}{3\lambda}}} & \left( {{Expression}\quad 6} \right) \\\left( {1\text{-}3} \right)^{\prime} & \quad \\{L = {\left( {p \pm \frac{1}{N}} \right)\frac{4n_{0}W_{0}^{2}}{3\lambda}}} & \left( {{Expression}\quad 7} \right)\end{matrix}$

(2) Symmetrical Center Incidence

In a slab waveguide having the uniform refractive index n₀, with respectto an incident beam that is incident symmetrically with respect to thecenter in the direction of the width, an exiting beam having the sameprofile as the incident beam exits from the exit surface with theposition and number thereof being varied according to the slab length Las shown by the following (Expression 8). Here, p and N are integers.Moreover, the integer p is an integer where (p±1/N) is positive.$\begin{matrix}{L = {\left( {p \pm \frac{1}{N}} \right)\frac{n_{0}W_{0}^{2}}{\lambda}}} & \left( {{Expression}\quad 8} \right)\end{matrix}$

By the slab length L satisfying (Expression 8), a number, N, of exitingbeams exit at intervals of W₀/N so as to be symmetrical with respect tothe center in the direction of the width of the exit surface.

When there is a plurality of incident beams, exiting beams areseparately obtained for each of the incident beams, and thecorresponding exiting beams are superimposed on each other.

The beam transmitted within the central plane, in the direction of thethickness, of the graded index slab waveguide 101 according to the firstembodiment corresponds to a case where the refractive index is n₀ andN=2 in (Expression 8). Therefore, the incident beam is split into twobeams at the exit end.

On the other hand, the beam of (ii) not transmitted within the centralplane in the direction of the thickness propagates along the centralplane while meandering in the direction of the thickness as shown inFIG. 1B, because it is affected by the refractive index distribution inthe direction of the thickness. That is, since the beam traveling in adirection away from the central plane always travels from a part wherethe refractive index is relatively high to a part where the refractiveindex is relatively low, as the beam travels, the angle between thedirection of travel and the direction of the thickness graduallyincreases, and becomes 90° at the position farthest from the centralaxis. Moreover, since the beam traveling in a direction toward thecentral plane always travels from a part where the refractive index isrelatively low to a part where the refractive index is relatively high,as the beam travels, the angle between the direction of travel and thedirection of the thickness gradually decreases, and becomes smallest atthe position intersecting the central plane. Since the refractive indexthat affects the beam of (ii) not transmitted within the central planein the direction of the thickness is always lower than the refractiveindex at the center although it makes the beam meander, the speed of thebeam (ii) is higher than that of the beam of (i) transmitted on thecentral axis in the direction of the thickness.

When the refractive index distribution is the refractive indexdistribution of the quadratic function shown in (Expression 1), thecomponent of the transmission speed parallel to the central plane of thebeam of (ii) not transmitted within the central plane in the directionof the thickness is equal to the transmission speed of the beam of (i)transmitted within the central plane in the direction of the thickness.This means that there is no mode dispersion in the direction of thethickness. Therefore, the component, parallel to the central plane, ofthe beam of (ii) not transmitted within the central plane in thedirection of the thickness (component, in a direction vertical to thedirection of the thickness, of a meandering beam) of the incident beamis split into two beams symmetrically with respect to the center in thedirection of the width at the exit surface like the beam of (i)transmitted within the central plane in the direction of the thickness.

Since the component, vertical to the central plane, of the beam of (ii)not transmitted within the central plane in the direction of thethickness (component, in the direction of the thickness, of a meanderingbeam) of the incident beam changes according to the propagation positionof the meandering beam, the condition of the exiting beams cannot bedetermined. However, the component, in the direction of the thickness,of the meandering beam is not affected by a signal waveform disturbancedue to the mode dispersion, because the mode dispersion in the directionof the thickness does not occur. For this reason, the component behavesequivalently to that in the case where there is no influence of the modedispersion also in the direction of the width. Therefore, the twoexiting beams each have the same image as the incident beam. From theabove result, the beam of (ii) not transmitted within the central planein the direction of the thickness (meandering beam) is split into twobeams as the same image as the incident beam symmetrically with respectto the center in the direction of the width according to the slabwaveguide configuration like in the case of (i).

As described above, since the incident beam is equally split into twobeams with respect to all the eigenmodes in the direction of thethickness of the graded index slab waveguide 101, an optical device canbe obtained that functions, if the incident beam is incident on thecentral position, in the direction of the width, of the incidentsurface, as a 1×2 splitting device even when the incident beam isposition-shifted from the center in the direction of the thickness orhas a large spread angle. Since the position shift, from the center inthe direction of the width, of the incident beam is a cause of animbalance in the splitting ratio between the exiting beams, when it isintended to obtain equal exiting beams, it is preferable that positionshift be minimized. However, it is possible to adjust the splittingratio by actively using the position shift.

As described above, since the 1×2 splitting device according to thefirst embodiment has a graded index slab waveguide having a slab lengthL which is an odd multiple of a value expressed by the followingexpression, with respect to an incident beam that is incident on thecenter, in the direction of the width, of the incident surface of thegraded index slab waveguide, two exiting beams can be generated andoutputted symmetrically with respect to the center, in the direction ofthe width, of the exit surface. $\frac{n_{0}W_{0}^{2}}{2\lambda}$

By interchanging the incident surface and the exit surface, the 1×2optical splitting device according to the first embodiment can be usedas a 2×1 optical combining device. In this case, two incident beams aremade incident symmetrically with respect to the center, in the directionof the width, of the incident surface, and one exiting beam is made toexit from the center, in the direction of the width, of the exitsurface. The slab length L of the 2×1 optical combining device is equalto the slab length L of the 1×2 optical splitting device.

Second Embodiment

FIG. 2 is a perspective view showing the general outline of a gradedindex slab waveguide of an optical device that splits one beam intoeight beams according to a second embodiment of the present invention.The optical device according to the second embodiment comprises as amain element the graded index slab waveguide 201 that transmits beams.The graded index slab waveguide 201 is, as shown in FIG. 2, a sheet-formmulti-mode optical transmission line that extends parallel to the x-zplane. The graded index slab waveguide 201 has a refractive indexdistribution such that the highest refractive index n_(max) is providedat the center in the direction of the thickness and the refractive indexdoes not increase with distance from the center. The graded index slabwaveguide 201 has a uniform refractive index in the direction of thewidth and has no refractive index distribution. The graded index slabwaveguide 201 includes an incident surface 202 and an exit surface 203.The incident surface 202 is opposed to an incident portion (not shown)that makes an incident beam 207 incident on the central position in thedirection of the width. The exit surface 203 is opposed to a lightreceiving portion 220 that receives eight exiting beams 208 that exitfrom positions symmetrical with respect to the center in the directionof the width. Moreover, the optical device 200 according to the secondembodiment is provided with an array O/E converter 221 and an outputelectric line (bus) 222. The array O/E converter 221 includes eightlight receiving portions 220. The array O/E converter 221 is connectedto the output electric line 222.

In the second embodiment, the slab length L substantially coincides withn₀×W₀ ²/(8%), and the distance D1 between the exit positions of theeight exiting beams 208 substantially coincides with W₀/8. Here, theeffective refractive index of the 0th-order mode beam excited in thedirection of the width is n₀. By setting the slab length L and the exitposition distance D1 to these values, eight images that are the same asthe incident beam are formed on the exit surface at intervals ofsubstantially W₀/8 so as to be symmetrical with respect to the center inthe direction of the width. The graded index slab waveguide 201according to the second embodiment outputs the eight images formed onthe exit surface 203, as the eight exiting beams 208. The eight exitingbeams 208 which are outputs of the same image as the incident beam 207have the same profile. According to the self-imaging principle, since asimilar phenomenon occurs every length of the value of the expressionshown below, by changing the integer p, the slab length L can beadjusted according to use. The detailed mechanism of splitting one beaminto eight beams and the mechanism in which there is no signal beamwaveform disturbance in the direction of the thickness and in thedirection of the width even in the case of high-speed transmission aresimilar to those of the first embodiment.$\left( {p \pm \frac{1}{8}} \right)\frac{{nW}_{0}^{2}}{\lambda}$(p is an integer that makes the value inside the parentheses positive)

By the above structure, the incident beam 207 is incident, as a signalbeam, on the center, in the direction of the width, of the incidentsurface 202, and is transmitted inside the graded index slab waveguide201. The signal beam is split into eight beams according to theself-imaging principle of the MMI inside the graded index slab waveguide201, and exits as the eight exiting beams 208 from positions, apart fromeach other in the direction of the width, of the exit surface 203 toreach the eight light receiving portions 220. The signal beams receivedby the light receiving portions 220 are converted into electric signalsby the array O/E converter 221, and outputted to the outside from theoutput electric line (bus) 222.

Moreover, in the optical device according to the second embodiment, thelight receiving portions 220 that receive the exiting beams 208 havingexited from the exit surface are formed in the array O/E converter 221and the array O/E converter 221 is connected to the output electric line222. By this structure, the exiting beams outputted from the gradedindex slab waveguide 201 are converted into electric signals in aspace-saving manner, so that a coupler that couples the exiting beams toan optical fiber or the like is unnecessary. Consequently, the opticaldevice according to the second embodiment can be structured so as to beeasy to adjust and compact.

As described above, since the 1×8 optical splitting device according tothe second embodiment has a graded index slab waveguide having a slablength L of a value expressed by the following expression, with respectto an incident beam that is incident on the center, in the direction ofthe width, of the incident surface of the graded index slab waveguide,eight exiting beams can be generated and outputted symmetrically withrespect to the center, in the direction of the width, of the exitsurface.$\left( {p \pm \frac{1}{8}} \right)\frac{{nW}_{0}^{2}}{\lambda}$(p is an integer that makes the value inside the parentheses positive)

By interchanging the incident surface and the exit surface, the 1×8optical splitting device according to the second embodiment can be usedas an 8×1 optical combining device. In this case, eight incident beamsare made incident symmetrically with respect to the center, in thedirection of the width, of the incident surface, and one exiting beam ismade to exit from the center, in the direction of the width, of the exitsurface. The slab length L of the 8×1 optical combining device is equalto the slab length L of the 1×2 optical splitting device.

While an example of the 1×2 optical splitting device is shown in thefirst embodiment and an example of the 1×8 optical splitting device isshown in the second embodiment, generally, a 1×N (N=1, 2, 3, . . . )optical splitting device can be similarly designed. In this case, bymaking incident one incident beam on the central position, in thedirection of the width, of the incident surface of the graded index slabwaveguide having a slab length L that satisfies the value of thefollowing expression, a number, N, of exiting beams can be obtained soas to be symmetrical with respect to the center, in the direction of thewidth, of the exit surface.$\left( {p \pm \frac{1}{N}} \right)\frac{{nW}_{0}^{2}}{\lambda}$(p is an integer that makes the value inside the parentheses positive)

In the case of an N×1 optical combining device, by making incident anumber, N, of incident beams symmetrically with respect to the center,in the direction of the width, of the incident surface of the gradedindex slab waveguide having a similar slab length, one exiting beam canbe obtained at the center of the exit surface.

Third Embodiment

FIG. 3 is a perspective view showing the general outline of a two-signalstraight sheet bus which is an optical device according to a thirdembodiment of the present invention. The optical device according to thethird embodiment comprises as a main element a graded index slabwaveguide 301 that transmits beams. The graded index slab waveguide 301is, as shown in FIG. 3, a sheet-form multi-mode optical transmissionline that extends parallel to the x-z plane. The graded index slabwaveguide 301 has a distribution such that the highest refractive indexn_(max) is provided at the center in the direction of the thickness andthe refractive index does not increase with distance from the center.The graded index slab waveguide 301 has a uniform refractive index inthe direction of the width and has no refractive index distribution. Theoptical device according to the third embodiment is provided with anarray E/O converter 332, an input electric line (bus) 333, and an arrayO/E converter 336 and an output electric line (bus) 337.

The array E/O converter 332 includes a first light emitting portion 330and a second light emitting portion 331. The first light emittingportion 330 makes a first beam 338 (wavelength: λ) incident on a givenposition in the direction of the width on an incident surface 302 of thegraded index slab waveguide 301. The second light emitting portion 331makes a second beam 339, having the same wavelength as the first beam,incident on a given position in the direction of the width on theincident surface of the graded index slab waveguide 301. Moreover, thearray E/O converter 332 is connected to the input electric line (bus)333. The array E/O converter 332 converts external electric signalsinputted from the input electric line 333 into signal beams emitted fromthe first light emitting portion 330 and the second light emittingportion 331.

The array O/E converter 336 includes a first light receiving portion 334and a second light receiving portion 335. The first light receivingportion 334 is disposed in a position whose position in the direction ofthe width is the same as that of the first light emitting portion 330 onthe exit surface 303 of the graded index slab waveguide 301. The secondlight receiving portion 335 is disposed in a position whose position inthe direction of the width is the same as that of the second lightemitting portion 331 on the exit surface of the graded index slabwaveguide 301. Moreover, the array O/E converter 336 is connected to theoutput electric line (bus) 337. The array O/E converter 336 convertssignal beams received by the first light receiving portion 334 and thesecond light receiving portion 335 into electric signals, and outputsthe electric signals to the output electric line 337.

The slab length L of the graded index slab waveguide 301 substantiallycoincides with 8×n₀×W₀ ²/λ. Here, the effective refractive index of the0th-order mode beam excited in the direction of the width is n₀. Theslab length L of the graded index slab waveguide 301 corresponds to thecase where a plurality of incident beams is superimposed one on anotherin (Expression 2) of (1) Asymmetrical incidence described in the firstembodiment. By setting the slab length L like this, the first beam 338incident from the first light emitting portion 330 forms an image havingthe same profile as that when the beam is incident in the vicinity ofthe first light receiving portion 334. Likewise, the second beam 339incident from the second light emitting portion 331 forms an imagehaving the same profile as that when the beam is incident in thevicinity of the first light receiving portion 335. According to theself-imaging principle, since a similar phenomenon occurs every lengthof 8×n₀×W₀ ²/λ, by setting the slab length L to an integral multiple of8×n₀×W₀ ²/λ, the length of the graded index slab waveguide 301 can beadjusted according to use. The detailed mechanism of splitting and themechanism in which there is no signal beam waveform disturbance in thedirection of the thickness and in the direction of the width even in thecase of high-speed transmission are similar to those of the firstembodiment.

By the above structure, when an external electric signal is inputted tothe array E/O converter 332 from the input electric line 333, the arrayE/O converter 332 converts the external electric signal into the firstbeam 338 emitted from the first light emitting portion 330 and thesecond beam 339 emitted from the second light emitting portion 331. Thefirst beam 338 emitted from the first light emitting portion 330 isincident on the graded index slab waveguide 301 through the incidentsurface 302 to be transmitted. The first beam 338 forms, according tothe self-imaging principle, an image having the same profile as thatwhen the beam is incident in the vicinity of the first light receivingportion 334. By this, the first beam 338 is outputted from the exitsurface 303 to the first light receiving portion 334. On the other hand,the second beam 339 emitted from the second light emitting portion 331is incident on the graded index slab waveguide 301 through the incidentsurface 302 to be transmitted. The second beam 339 forms, according tothe self-imaging principle, an image having the same profile as thatwhen the beam is incident in the vicinity of the second light receivingportion 335. By this, the second beam 339 is outputted from the exitsurface 303 to the first light receiving portion 335. The first lightreceiving portion 334 outputs an electric signal corresponding to thereceived first beam 338. The second light receiving portion 335 outputsan electric signal corresponding to the received second beam 339. Theoutputted electric signals are outputted to the outside from the outputelectric line 337. As described above, by using the MMI, it isunnecessary to provide separate optical waveguides to straightlytransmit two signal beams, and two signal beams can be independentlytransmitted with one graded index slab waveguide 301.

FIG. 4 is a perspective view showing the general outline of aneight-signal straight sheet bus which is an optical device according toa modification of the third embodiment of the present invention. Theschematic structure of the optical device of the modification is thesame as that of the previously-described two-signal straight sheet bus.The optical device of the modification is provided with a graded indexslab waveguide 401, an array E/O converter 432, an input electric line(bus) 333, an array O/E converter 436 and an output electric line (bus)337. The array E/O converter 432 has generally the same structure as thearray E/O converter 332 of the two-signal straight sheet bus; however,it is different in that a light emitting portion group 446 comprisingeight light emitting portions is formed instead of the first lightemitting portion 330 and the second light emitting portion 331.Moreover, the array O/E converter 436 has generally the same structureas the array O/E converter 336 of the two-signal straight sheet bus;however, it is different in that a light receiving portion group 447comprising eight light receiving portions is formed instead of the firstlight receiving portion 334 and the second light receiving portion 335.The positions, in the direction of the width, of the light emittingportions included in the light emitting portion group 446 all correspondto those of the light receiving portions included in the light receivingportion group 447.

The light emitting portion group 446 makes a first beam 438 to an eighthbeam 445, which are eight signal beams all having the same wavelength,independently incident on the graded index slab waveguide 401 through anincident surface 402 based on the external electric signal inputted fromthe input electric line 333. The graded index slab waveguide 401transmits the first beam 438 to the eighth beam 445. The first beam 438to the eighth beam 445 exit from an exit surface 403 and are received bythe light receiving portions, whose positions in the direction of thewidth are the same, of the light receiving portion group 447 like in thecase of the graded index slab waveguide 301. The principle that eightincident beams independently appear in parallel positions in thedirection of the width at the exit end corresponds to the case where aplurality of incident beams is superimposed one on another in(Expression 2) of (1) Asymmetrical incidence described in the firstembodiment. As described above, by using the MMI, it is unnecessary toprovide separate optical waveguides to straightly transmit eight signalbeams, and eight signal beams can be independently transmitted with onegraded index slab waveguide 401.

While examples of the two-signal straight sheet bus and the eight-signalstraight sheet bus are shown in the third embodiment, generally, anN-signal straight sheet bus (N=1, 2, 3, . . . ) can be similarlydesigned. In this case, by making a number, N, of incident beamsincident on given positions of the incident surface of the graded indexslab waveguide having a slab length L which is substantially an integralmultiple of the following expression, a number, N, of exiting beams canbe obtained from positions, whose positions in the direction of thewidth are the same, of the exit surface:$\frac{8n_{0}W_{0}^{2}}{\lambda}$

Fourth Embodiment

FIG. 5 is a perspective view showing the general outline of a two-signalcross sheet bus which is an optical device according to a fourthembodiment of the present invention. The optical device according to thefourth embodiment comprises as a main element a graded index slabwaveguide 501 that transmits beams. The graded index slab waveguide 501is, as shown in FIG. 5, a sheet-form multi-mode optical transmissionline that extends parallel to the x-z plane. The graded index slabwaveguide 501 has a distribution such that the highest refractive indexn_(max) is provided at the center in the direction of the thickness andthe refractive index does not increase with distance from the center.The graded index slab waveguide 501 has a uniform refractive index inthe direction of the width and has no refractive index distribution. Theoptical device according to the fifth embodiment is provided with anarray E/O converter 532, an input electric line (bus) 333, and an arrayO/E converter 536 and an output electric line (bus) 337.

The array E/O converter 532 includes a first light emitting portion 530and a second light emitting portion 531. The first light emittingportion 530 makes a first beam 538 (wavelength: λ) incident on a givenposition in the direction of the width on the incident surface of thegraded index slab waveguide 501. The second light emitting portion 531makes a second beam 539, having the same wavelength as the first beam,incident on a given position in the direction of the width on theincident surface of the graded index slab waveguide 501. Moreover, thearray E/O converter 532 is connected to the input electric line (bus)333. The array E/O converter 532 converts external electric signalsinputted from the input electric line 333 into signal beams emitted fromthe first light emitting portion 530 and the second light emittingportion 531.

The array O/E converter 536 includes a first light receiving portion 534and a second light receiving portion 535. The first light receivingportion 534 is disposed in a position symmetrical to the second lightemitting portion 531 with respect to the center in the direction of thewidth on the exit surface of the graded index slab waveguide 501. Thesecond light receiving portion 535 is disposed in a position symmetricalto the first light emitting portion 530 with respect to the center inthe direction of the width on the exit surface of the graded index slabwaveguide 501. Moreover, the array O/E converter 536 is connected to theoutput electric line (bus) 337. The array O/E converter 536 convertssignal beams received by the first light receiving portion 534 and thesecond light receiving portion 535 into electric signals, and outputsthe electric signals to the output electric line 337.

The slab length L of the graded index slab waveguide 501 substantiallycoincides with 4×n₀×W₀ ²/λ. Here, the effective refractive index of the0th-order mode beam excited in the direction of the width is n₀. Theslab length L of the graded index slab waveguide 501 corresponds to thecase where a plurality of incident beams is superimposed one on anotherin (Expression 3) of (1) Asymmetrical incidence described in the firstembodiment. By setting the slab length L like this, the first beam 538emitted from the first light emitting portion 530 and incident throughan incident surface 502 forms an image having the same profile as thatwhen the beam is incident in the vicinity of the first light receivingportion 534. Likewise, the second beam 539 emitted from the second lightemitting portion 531 and incident through the incident surface 502 formsan image having the same profile as that when the beam is incident inthe vicinity of the second light receiving portion 535. According to theself-imaging principle, since a similar phenomenon occurs every lengthof 8×n₀×W₀ ²/λ, by setting the slab length L to an odd multiple of4×n₀×W₀ ²/λ, the length of the graded index slab waveguide 501 can beadjusted according to use. The detailed mechanism of splitting and themechanism in which there is no signal beam waveform disturbance in thedirection of the thickness and in the direction of the width even in thecase of high-speed transmission are similar to those of the firstembodiment.

By the above structure, when an external electric signal is inputted tothe array E/O converter 532 from the input electric line 333, the arrayE/O converter 532 converts the external electric signal into the firstbeam 538 emitted from the first light emitting portion 530 and thesecond beam 539 emitted from the second light emitting portion 531. Thefirst beam 538 emitted from the first light emitting portion 530 isincident on the graded index slab waveguide 501 through the incidentsurface 502 to be transmitted. The first beam 538 forms, according tothe self-imaging principle, an image having the same profile as thatwhen the beam is incident in the vicinity of the first light receivingportion 534. By this, the first beam 538 exits from the exit surface 503and is received by the first light receiving portion 534. On the otherhand, the emitted second beam 539 is incident on the graded index slabwaveguide 501 through the incident surface 502 to be transmitted. Thesecond beam 539 forms, according to the self-imaging principle, an imagehaving the same profile as that when the beam is incident in thevicinity of the second light receiving portion 535. By this, the secondbeam 539 exits from the exit surface 503 and is outputted by the firstlight receiving portion 535. The first light receiving portion 534outputs an electric signal corresponding to the received first beam 538.The second light receiving portion 535 outputs an electric signalcorresponding to the received second beam 539. The outputted electricsignals are outputted to the outside from the output electric line 337.As described above, by using the MMI, it is unnecessary to provideseparate optical waveguides to transmit two signal beams so as to crosseach other, and two signal beams can be independently transmitted withone graded index slab waveguide 501.

FIG. 6 is a perspective view showing the general outline of aneight-signal cross sheet bus which is an optical device according to amodification of the fourth embodiment of the present invention. Theschematic structure of the optical device of the modification is thesame as that of the previously-described two-signal straight sheet bus.The optical device of the modification is provided with a graded indexslab waveguide 601, an array E/O converter 632, an input electric line(bus) 333, an array O/E converter 636 and an output electric line (bus)337. The array E/O converter 632 has generally the same structure as thearray E/O converter 532 of the two-signal straight sheet bus; however,it is different in that a light emitting portion group 646 comprisingeight light emitting portions is formed instead of the first lightemitting portion 530 and the second light emitting portion 531.Moreover, the array O/E converter 636 has generally the same structureas the array O/E converter 536 of the two-signal straight sheet bus;however, it is different in that a light receiving portion group 647comprising eight light receiving portions is formed instead of the firstlight receiving portion 534 and the second light receiving portion 535.The light emitting portions included in the light emitting portion group640 are all disposed in positions symmetrical to the light receivingportions included in the light receiving portion group 647 with respectto the center in the direction of the width.

The light emitting portion group 646 makes a first beam 638 to an eighthbeam 645, which are eight signal beams all having the same wavelength,independently incident on the graded index slab waveguide 601 through anincident surface 502 based on the external electric signal inputted fromthe input electric line 333. The graded index slab waveguide 601transmits the first beam 638 to the eighth beam 645. The first beam 638to the eighth beam 645 exit from an exit surface 603 and are outputtedfrom the light receiving portions, symmetrical with respect to thecenter in the direction of the width, in the light receiving portiongroup 647 like in the case of the graded index slab waveguide 501. Theprinciple that eight incident beams independently appear in parallelpositions in the direction of the width at the exit end corresponds tothe case where a plurality of incident beams is superimposed one onanother in (Expression 3) of (1) Asymmetrical incidence described in thefirst embodiment. As described above, by using the MMI, it isunnecessary to provide separate optical waveguides to transmit eightsignal beams so as to cross each other, and eight signal beams can beindependently transmitted with one graded index slab waveguide 601.

While examples of the two-signal cross sheet bus and the eight-signalcross sheet bus are shown in the third embodiment, generally, anN-signal cross sheet bus (N=1, 2, 3, . . . ) can be similarly designed.In this case, by making a number, N, of incident beams incident on givenpositions of the incident surface of the graded index slab waveguidehaving a slab length L which is substantially an odd multiple of thefollowing expression, a number, N, of exiting beams can be obtained frompositions the same as the positions symmetrical with respect to thecenter, in the direction of the width, of the exit surface.$\frac{4n_{0}W_{0}^{2}}{\lambda}$

Fifth Embodiment

FIG. 7 is a perspective view showing the general outline of a two-signalstar coupler which is an optical device according to a fifth embodimentof the present invention. The optical device according to the fifthembodiment comprises as a main element a graded index slab waveguide 701that transmits beams. The graded index slab waveguide 701 is, as shownin FIG. 7, a sheet-form multi-mode optical transmission line thatextends parallel to the x-z plane. The graded index slab waveguide 701has a distribution such that the highest refractive index n_(max) isprovided at the center in the direction of the thickness and therefractive index does not increase with distance from the center. Thegraded index slab waveguide 701 has a uniform refractive index in thedirection of the width and has no refractive index distribution. Theoptical device according to the fifth embodiment is provided with anarray E/O converter 732, an input electric line (bus) 333, an array O/Econverter 736 and an output electric line (bus) 337.

The array E/O converter 732 includes a first light emitting portion 730and a second light emitting portion 731. The first light emittingportion 730 makes a first beam 738 (wavelength: λ) incident on aposition a predetermined distance away from the center in the directionof the width on the incident surface of the graded index slab waveguide701. The second light emitting portion 731 makes a second beam 739,having the same wavelength as the first beam, incident on a positionsymmetrical to the first light emitting portion 703 with respect to thecenter in the direction of the width on the incident surface of thegraded index slab waveguide 701. Moreover, the array E/O converter 732is connected to the input electric line (bus) 333. The array E/Oconverter 732 converts external electric signals inputted from the inputelectric line 333 into signal beams emitted from the first lightemitting portion 730 and the second light emitting portion 731.

The array O/E converter 736 includes a first light receiving portion 734and a second light receiving portion 735. The first light receivingportion 734 is disposed in a position whose position in the direction ofthe width is the same as that of the first light emitting portion 730 onthe exit surface of the graded index slab waveguide 701. The secondlight receiving portion 735 is disposed in a position whose position inthe direction of the width is the same as that of the second lightemitting portion 731 on the exit surface of the graded index slabwaveguide 701. Moreover, the array O/E converter 736 is connected to theoutput electric line (bus) 337. The array O/E converter 736 convertssignal beams received by the first light receiving portion 734 and thesecond light receiving portion 735 into electric signals, and outputsthe electric signals to the output electric line 337.

The slab length L of the graded index slab waveguide 701 substantiallycoincides with 2×n₀×W₀ ²/λ. Here, the effective refractive index of the0th-order mode beam excited in the direction of the width is n₀. Theslab length L of the graded index slab waveguide 701 corresponds to thecase where a plurality of incident beams is superimposed one on anotherwhen p=O and N=2 in (Expression 4) of (1) Asymmetrical incidencedescribed in the first embodiment. By setting the slab length L likethis, the first beam 738 emitted from the first light emitting portion730 and incident through an incident surface 702 forms two images havingthe same profile as that when the beam is incident in the vicinity ofthe first light receiving portion 734 and the second light receivingportion 735. Likewise, the second beam 739 emitted from the second lightemitting portion 731 and incident through the incident surface 702 formstwo images having the same profile as that when the beam is incident inthe vicinity of the first light receiving portion 734 and the secondlight receiving portion 735. According to the self-imaging principle,since a similar phenomenon occurs every length of the value of theexpression shown below, by changing the value of p in (Expression 10) ofthe slab length L, the length of the graded index slab waveguide 701 canbe adjusted according to use.$\left( {p \pm \frac{1}{2}} \right)\frac{4n_{0}W_{0}^{2}}{\lambda}$(p is an integer that makes the value inside the parentheses positive)

The detailed mechanism of splitting and the mechanism in which there isno signal beam waveform disturbance in the direction of the thicknessand in the direction of the width even in the case of high-speedtransmission are similar to those of the first embodiment.

By the above structure, when an external electric signal is inputted tothe array E/O converter 732 from the input electric line 333, the arrayE/O converter 732 converts the external electric signal into the firstbeam 738 emitted from the first light emitting portion 730 and thesecond beam 739 emitted from the second light emitting portion 731. Thefirst beam 738 emitted from the first light emitting portion 730 isincident on the graded index slab waveguide 701 through the incidentsurface 702 to be transmitted. The first beam 538 forms, according tothe self-imaging principle, two images having the same profile as thatwhen the beam is incident in the vicinity of the first light receivingportion 734 and the second light receiving portion 735. By this, thefirst beam 738 exits from the exit surface 703 and is outputted to thefirst light receiving portion 734 and the second light receiving portion735. On the other hand, the second beam 739 emitted from the secondlight emitting portion 731 is incident on the graded index slabwaveguide 701 through the incident surface 702 to be transmitted. Thesecond beam 739 forms, according to the self-imaging principle, twoimages having the same profile as that when the beam is incident in thevicinity of the first light receiving portion 734 and the second lightreceiving portion 735. By this, the second beam 739 exits from the exitsurface 703 and is outputted to the first light receiving portion 735.The first light receiving portion 734 outputs an electric signalcorresponding to the received first beam 738. The second light receivingportion 735 outputs an electric signal corresponding to the receivedsecond beam 739. The outputted electric signals are outputted to theoutside from the output electric line 337. As described above, by usingthe MMI, a two-signal star coupler can be realized with one graded indexslab waveguide 701.

While the above two-signal star coupler is an optical device that makestwo signal beams incident on positions symmetrical in the direction ofthe width and makes the two signal beams exit from positions symmetricalin the direction of the width, not less than two signal beams may beinputted and outputted. In the case of a star coupler that makes anumber, N (N is an even number), of signal beams incident on positionssymmetrical in the direction of the width and makes a number, N, ofsignal beams exit from positions symmetrical in the direction of thewidth, the slab length L of the graded index slab waveguide satisfiesthe following expression:$\left( {p \pm \frac{1}{N_{EVEN}}} \right)\frac{4n_{0}W_{0}^{2}}{\lambda}$(p is an integer that makes the value inside the parentheses positive,N_(EVEN)=2, 4, 6, . . . )Since p is an integer in the above expression, by changing p, the slablength L of the graded index slab waveguide can be adjusted to a desiredlength. In particular, when it is unnecessary to adjust the length, byp=0, a graded index slab waveguide with a minimum length can beobtained.

On the other hand, in the case of a star coupler that makes a number, N(N is an odd number), of signal beams incident and makes a number, N, ofsignal beams exit, the slab length L of the graded index slab waveguidesatisfies the value of the following expression:$\left( {p \pm \frac{1}{N_{ODD}}} \right)\frac{4n_{0}W_{0}^{2}}{\lambda}$(p is an integer that makes the value inside the parentheses positive,N_(ODD)=3, 5, 7, . . . )The above expression is the same in format as that in the case of aplurality of signal beams. However, when N is an odd number, thepositions of the incident and exiting signal beams are not symmetricalin the direction of the width. FIG. 19 is a top view showing a gradedindex slab waveguide of a star coupler having three input and outputbeams according to the present invention. In FIG. 19, a first beam 1902,a second beam 1903 and a third beam 1904 all having the same wavelengthλ are incident on the incident surface of a graded index slab waveguide1901. The first beam 1902 is incident on a position a distance X awayfrom one surface, parallel to the direction of the length, of the gradedindex slab waveguide 1901. The second beam 1903 is incident on aposition further the distance X away toward the one surface with respectto a position a distance 2W/3 (W is the slab width) away from the onesurface, parallel to the direction of the length, of the graded indexslab waveguide 1901. The third beam 1904 is incident on a positionfurther the distance X away toward the other surface with respect to aposition the distance 2W/3 (W is the slab width) away from the onesurface, parallel to the direction of the length, of the graded indexslab waveguide 1901.

When the three signal beams having the above incident positions areincident through the incident surface and transmitted in a graded indexslab waveguide having a slab length L satisfying the above-mentioned(Expression 12), the first beam 1902, the second beam 1903 and the thirdbeam 1904 form three images having the same profile as that when thebeams are incident on the following three positions, respectively: aposition the distance X away from the other surface, parallel to thedirection of the length, of the graded index slab waveguide 1901; aposition further the distance X away toward the other surface withrespect to a position a distance W/3 (W is the slab width) away from theother surface, parallel to the direction of the length, of the gradedindex slab waveguide 1901; and a position further the distance X awaytoward the one surface. Therefore, by forming light receiving portionsin the positions of the images of the signals, the exiting beams can beoutputted.

While the above example is a case where N=3, when N is an odd number motless than 3, a star coupler can also be structured that obtains outputsasymmetrical with respect to the center in the direction of the width inaccordance with the input of signal beams asymmetrical with respect tothe direction of the width by a graded index slab waveguide satisfying(Expression 12). As described above, when N is an odd number, a starcoupler can also be realized by using the self-imaging principle of theMMI. Since p is an integer in the above-mentioned (Expression 12), bychanging p, the slab length L of the graded index slab waveguide can beadjusted to a desired length. In particular, when it is unnecessary toadjust the length, by p=0, a graded index slab waveguide with a minimumlength can be obtained.

Sixth Embodiment

FIG. 8A is a perspective view showing the general outline of a one sidecontrol type optical switch which is an optical device according to asixth embodiment of the present invention. The optical device accordingto the sixth embodiment comprises as main elements a first graded indexslab partial waveguide 801 a and a second graded index slab partialwaveguide 801 b that transmit beams. The first graded index slab partialwaveguide 801 a and the second graded index slab partial waveguide 801 bare both sheet-form multi-mode optical transmission lines that extendparallel to the x-z plane as shown in FIG. 8A. The first graded indexslab partial waveguide 801 a and the second graded index slab partialwaveguide 801 b have the same size in the direction of the width. Thefirst graded index slab partial waveguide 801 a and the second gradedindex slab partial waveguide 801 b are disposed so as to be connectedcontinuously in the direction of the width. Moreover, the first gradedindex slab partial waveguide 801 a and the second graded index slabpartial waveguide 801 b are made of a polymer exhibiting a predeterminedthermooptic effect.

The first graded index slab partial waveguide 801 a has an incidentposition for making an incident beam 804 incident on an incident surface801 and an exit position for making an exiting beam 809 exit on an exitsurface 802. The incident position and the exit position are situated inpositions away from the center in the direction of the width when thefirst graded index slab partial waveguide 801 a and the second gradedindex slab partial waveguide 801 b are regarded as one slab waveguide801, and their positions in the direction of the width are the same. Thesecond graded index slab partial waveguide 801 b has an exit positionfor making an exiting beam 808 exit on the exit surface 802. The exitposition of the second graded index slab partial waveguide 801 b issituated in a position symmetrical to the exit position of the firstgraded index slab partial waveguide 801 a with respect to the center inthe direction of the width when the first graded index slab partialwaveguide 801 a and the second graded index slab partial waveguide 801 bare regarded as one slab waveguide. Moreover, the second graded indexslab partial waveguide 801 b has a thermal sheet 805 on the top surface.The thermal sheet 805 is connected to a temperature controller 807 by aconnection line 806.

The temperature controller 807 controls the temperature of the thermalsheet 805 based on the externally supplied control signal. Since thetemperature of the second graded index slab partial waveguide 801 b ischanged by controlling the temperature of the thermal sheet 805, theabsolute value of the refractive index is changed based on thethermooptic effect. The optical device of the sixth embodiment isstructured so that when the temperature controller 807 is OFF, therefractive index distributions of the first graded index slab partialwaveguide 801 a and the second graded index slab partial waveguide 801 bcoincide with each other. At this time, the first graded index slabpartial waveguide 801 a and the second graded index slab partialwaveguide 801 b both have a distribution such that the highestrefractive index n_(max) is provided at the center in the direction ofthe thickness and the refractive index does not increase with distancefrom the center. Moreover, at this time, the first graded index slabpartial waveguide 801 a and the second graded index slab partialwaveguide 801 b have a uniform refractive index in the direction of thewidth and have no refractive index distribution. The optical device ofthe sixth embodiment is structured so that when the temperaturecontroller 807 is ON, the highest refractive index of the first gradedindex slab partial waveguide 801 a is higher than the absolute value ofthe highest refractive index of the second graded index slab partialwaveguide 801 b and their overall refractive index distributions aredifferent from each other.

The slab lengths L of the first graded index slab partial waveguide 801a and the second graded index slab partial waveguide 801 b substantiallycoincide with 4×n₀×W₀ ²/λ. Here, the effective refractive index of the0th-order mode beam excited in the direction of the width is n₀. Theslab lengths L of the first graded index slab partial waveguide 801 aand the second graded index slab partial waveguide 801 b correspond tothe case where a plurality of incident beams is superimposed one onanother in (Expression 3) of (1) Asymmetrical incidence described in thefirst embodiment. According to the self-imaging principle, since asimilar phenomenon occurs every length of 8×n₀×W₀ ²/λ, by setting theslab lengths L to an integral multiple of 4×n₀×W₀ ²/λ, the lengths ofthe first graded index slab partial waveguide 801 a and the secondgraded index slab partial waveguide 801 b can be adjusted according touse.

Next, the mechanism of the optical switch that changes the beam pathdirection by the above-described structure will be described. When thetemperature controller 807 is OFF, the incident beam 804 incident on theincident position of the first graded index slab partial waveguide 801 ais transmitted with the first graded index slab partial waveguide 801 aand the second graded index slab partial waveguide 801 b as one slabwaveguide since these waveguides have the same refractive indexdistribution. Therefore, according to (Expression 4) of (1) Asymmetricalincidence described in the first embodiment, an image having the sameprofile as the incident beam is formed in the exit position whoseposition in the direction of the width on the exit surface 803 issymmetrical to the incident position with respect to the center. Thisimage exits as the exiting beam 808.

On the other hand, when the temperature controller 807 is ON, since thehighest refractive index of the first graded index slab partialwaveguide 801 a is higher than the highest refractive index of thesecond graded index slab partial waveguide 801 b, by satisfying apredetermined refractive index difference, the second graded index slabpartial waveguide 801 b functions as cladding for the first graded indexslab partial waveguide 801 a. Therefore, the incident beam 804 incidenton the incident position of the first graded index slab partialwaveguide 801 a is trapped in the first graded index slab partialwaveguide 801 a to the exit surface 803 by the total reflection at theinterface between the first graded index slab partial waveguide 801 aand the second graded index slab partial waveguide 801 b. Consequently,the exiting beam 808 exits from the exit surface 803.

As described above, an optical switch capable of changing the traveldirection of the incident beam 804 can be realized by the ON-OFFswitching control of the temperature controller 807. While the opticaldevice of the sixth embodiment is an example structured so that therefractive index distributions of the first graded index slab partialwaveguide 801 a and the second graded index slab partial waveguide 801 bcoincide with each other when the temperature controller 807 is OFF, itmay be structured so that the refractive index distributions coincidewith each other when the temperature controller 807 is ON. In this case,when the temperature controller 807 is OFF, the highest refractive indexof the first graded index slab partial waveguide 801 a is lower than thehighest refractive index of the second graded index slab partialwaveguide 801 b, and their refractive index distributions are differentfrom each other. In a case where the case of this structure is adopted,when the temperature controller 807 is ON, the exiting beam 808 exitsfrom the second graded index slab partial waveguide 801 b, and when thetemperature controller 807 is OFF, the exiting beam 809 exits from thefirst graded index slab partial waveguide 801 a.

FIG. 8B is a perspective view showing the general outline of a both sidecontrol type optical switch which is an optical device according to afirst modification of the sixth embodiment of the present invention.Since the schematic structure of the first modification of the sixthembodiment is the same as that of the previously-described opticaldevice, only different parts will be described. The same referencenumerals indicate the same elements.

In the first modification of the sixth embodiment, the second gradedindex slab partial waveguide 801 b has the thermal sheet 805, and isconnected to the temperature controller 807 by the connection line 806.The optical device of the first modification of the sixth embodiment isstructured so that when the temperature controller 807 is OFF, therefractive index distributions of the first graded index slab partialwaveguide 801 a and the second graded index slab partial waveguide 801 bcoincide with each other and that the optical device of the firstmodification is structured so that when the temperature controller 807is ON, the temperatures of the first graded index slab partial waveguide801 a and the second graded index slab partial waveguide 801 b arecontrolled in opposite phase to increase the highest refractive index ofthe first graded index slab partial waveguide 801 a and decrease thehighest refractive index of the second graded index slab partialwaveguide 801 b so that their refractive index distributions aredifferent from each other. By structuring the optical device like this,switching can be performed at a higher speed than in the case of the oneside control type. In the first modification of the sixth embodiment,the optical device may be structured so that when the temperaturecontroller 807 is ON, the refractive index distributions of the firstgraded index slab partial waveguide 801 a and the second graded indexslab partial waveguide 801 b coincide with each other. In this case, theoptical device is structured so that when the temperature controller 807is OFF, the temperatures of the first graded index slab partialwaveguide 801 a and the second graded index slab partial waveguide 801 bare controlled in opposite phase to increase the highest refractiveindex of the first graded index slab partial waveguide 801 a anddecrease the highest refractive index of the second graded index slabpartial waveguide 801 b so that their refractive index distributions aredifferent from each other.

FIG. 20A is a perspective view showing the general outline of a one sidecontrol type optical switch which is an optical device according to asecond modification of the sixth embodiment of the present invention.Since the schematic structure of the second modification of the sixthembodiment is the same as that of the previously-described opticaldevice of the sixth embodiment, only different parts will be described.The same reference numerals indicate the same elements.

In the second modification of the sixth embodiment, the size, in thedirection of the width, of a first graded index slab partial waveguide2001 a is 1/{square root}{square root over ( )}2 times that when thefirst graded index slab partial waveguide 2001 a and a second gradedindex slab partial waveguide 2001 b are regarded as one opticalwaveguide. Moreover, in the second modification of the sixth embodiment,the optical device is structured so that when the temperature controller807 is ON, the highest refractive index of the first graded index slabpartial waveguide 801 a is higher than the highest refractive index ofthe second graded index slab partial waveguide 801 b and their overallrefractive index distributions are different from each other. Bystructuring the optical device like this, when the temperaturecontroller 807 is ON, the first graded index slab partial waveguide 2001a according to the second modification of the sixth embodiment functionsas a graded index slab waveguide whose size in the direction of thewidth is 1/{square root}{square root over ( )}2 W, and the size of thebasic mode is also 1/{square root}{square root over ( )}2 W₀. When theslab length L of the first graded index slab partial waveguide 2001 asubstantially coincides with 4×n₀×W₀ ²/λ, the condition is satisfied forthe exiting beam 809 to form an image having the same profile as theincident beam 804 in the exit position whose position in the directionof the width is the same as the incident position on the exit surface2003. Consequently, the optical device according to the secondmodification of the sixth embodiment is capable of generating an exitingbeam having the same profile as the incident beam based on theself-imaging principle of the multi-mode interference even when thetemperature controller 807 is ON.

While the optical device according to the second modification of thesixth embodiment is an example structured so that the refractive indexdistributions of the first graded index slab partial waveguide 2001 aand the second graded index slab partial waveguide 2001 b coincide witheach other when the temperature controller 807 is OFF, it is to be notedthat the optical device may be structured so that the refractive indexdistributions coincide with each other when the temperature controller807 is ON. In a case where the case of this structure is adopted, whenthe temperature controller 807 is ON, the exiting beam 808 exits fromthe second graded index slab partial waveguide 2001 b, and when thetemperature controller 807 is OFF, the exiting beam 809 exits from thefirst graded index slab partial waveguide 2001 a.

FIG. 20B is a perspective view showing the general outline of a bothside control type optical switch which is an optical device according toa third modification of the sixth embodiment of the present invention.The second modification of the sixth embodiment is an optical devicecomprising a combination of the previously-described first modificationand second modification of the sixth embodiment. The same referencenumerals indicate the same elements.

In the optical device according to the third modification of the sixthembodiment, the size, in the direction of the width, of the first gradedindex slab partial waveguide 2001 a is 1/{square root}{square root over( )}2 times that when the first graded index slab partial waveguide 2001a and the second graded index slab partial waveguide 2001 b are regardedas one optical waveguide, and the second graded index slab partialwaveguide 2001 b is also connected to the temperature controller 807 bythe connection line 806. By this structure, ON-OFF switching controlbased on the signal from the temperature controller 807 can be performedat high speed, and an exiting beam generated by the self-imagingprinciple of the multi-mode interference can be obtained in both the ONstate and the OFF state.

While examples in which refractive index control is performed by using apolymer with a high thermooptic effect are shown in all the descriptionsof the sixth embodiment, any method that independently changes therefractive indices of the first and second graded index slab partialwaveguides such as the electrooptic effect may be used.

Seventh Embodiment

FIG. 9 is a perspective view showing the general outline of an opticalswitch array which is an optical integrated device according to aseventh embodiment of the present invention. The optical switch array ofthe seventh embodiment is provided with a laminated optical switch group901 in which eight optical switches that are described in the firstmodification of the sixth embodiment (FIG. 8B) are laminated in thedirection of the thickness of the graded index slab waveguide 801. Ineach of the optical switches included in the laminated optical switchgroup 901, the part corresponding to the first graded index slab partialwaveguide 801 a described in the sixth embodiment is disposed on thelower side in the figure. The optical integrated device according to theseventh embodiment includes a first array O/E converter 905, a secondarray O/E converter 906, a first output electric line (bus) 907 and asecond output electric line (bus) 908.

The first array O/E converter 905 has a first light receiving portiongroup 903 comprising eight light receiving portions provided so as to beopposed to exit portions corresponding to the first graded index slabpartial waveguides 801 a of the optical switches. Moreover, the firstarray O/E converter 905 is connected to the first output signal line908. The second array O/E converter 906 has a second light receivingportion group 904 comprising eight light receiving portions provided soas to be opposed to exit portions corresponding to the second gradedindex slab partial waveguides 801 b of the optical switches. Moreover,the second array O/E converter 906 is connected to the second outputsignal line 908. In this example, the optical device is structured sothat when the temperature controller is ON, the exiting beam exits fromthe side of the first graded index slab partial waveguide and when thetemperature controller is ON, the exiting beam exits from the side ofthe first graded index slab partial waveguide.

In the above structure, an array incident beam 902 is made incident onthe parts (lower side in the figure) corresponding to the first gradedindex slab partial waveguides of the optical switches included in thelaminated optical switch 901, and is transmitted in the direction of thelength. The optical switches independently perform switching based onthe control of the temperature controller described in the sixthembodiment. When the temperature controller is ON, the exiting beamsfrom the optical switches are incident on the light receiving portionsincluded in the first light receiving portion group 903 of the firstarray O/E converter 905. Moreover, when the temperature controller isOFF, the exiting beams from the optical switches are incident on thelight receiving portions included in the second light receiving portiongroup 904 of the second array O/E converter 906.

The first array O/E converter 905 and the second array O/E converter 906assign a 1 signal when the exiting beams are made to exit to the lightreceiving portions, and assign a 0 signal when the exiting beams are notmade to exit to the light receiving portions. The signal assigned in thefirst array O/E converter 905 is outputted as an output signal to theoutside by the first output electric line 907. The signal assigned inthe second array O/E converter 906 is outputted as an output signal tothe outside by the second output electric line 908. As described above,by assigning signals, 8-digit digital signals and their inverted signalscan be parallelly transmitted.

As the array incident beam 902, a beam emitted from an array lightemitting device may be made directly incident or a beam from an externallight source may be made incident by an optical transmission linedisposed on the incident side of an optical fiber or the like. Moreover,the array incident beam may be generated by, for example, an opticaldevice that splits one beam into eight beams. Moreover, a heatproofmaterial or an insulating material such as air may be provided betweenthe switches of the integrated optical switch.

Eighth Embodiment

FIG. 10 is a perspective view showing the general outline of asingle-pair two-way straight sheet bus which is an optical deviceaccording to an eighth embodiment of the present invention. In theoptical device according to the eighth embodiment, one signal beamtransmission direction of the two-signal straight sheet bus described inthe third embodiment is reversed to enable two-way communication. Theoptical device according to the eighth embodiment comprises as a mainelement the graded index slab waveguide 1000. The graded index slabwaveguide 1001 is, as shown in FIG. 10, a sheet-form multi-mode opticaltransmission line that extends parallel to the x-z plane. The gradedindex slab waveguide 1001 has a distribution such that the highestrefractive index n_(max) is provided at the center in the direction ofthe thickness and the refractive index does not increase with distancefrom the center. The graded index slab waveguide 1001 has a uniformrefractive index in the direction of the width and has no refractiveindex distribution. The optical device according to the eighthembodiment is provided with a first E/O converter 1006, a second E/Oconverter 1009, a first O/E converter 1007, a second O/E converter 1008,a first input electric line (bus) 1010, a second input electric line(bus) 1011, a first output electric line (bus) 1012 and a second outputelectric line (bus) 1013.

The first E/O converter 1006 includes a first light emitting portion1014. The second E/O converter 1009 includes a second light emittingportion 1015. The first light emitting portion 1014 makes a second beam1019 (wavelength: λ) incident on a given position in the direction ofthe width on a first surface 1002 of the graded index slab waveguide1001. The second light emitting portion 1015 makes a first beam 1018having the same wavelength as the second beam 1019 incident on a givenposition in the direction of the width on a second surface 1003 of thegraded index slab waveguide 1001. The first E/O converter 1006 isconnected to the first input electric line (bus) 1010. The first E/Oconverter 1006 converts an external electric signal inputted from thefirst input electric line (bus) 1010 into a signal beam emitted from thefirst light emitting portion 1014. The second E/O converter 1009 isconnected to the second input electric line (bus) 1011. The second E/Oconverter 1009 converts an external electric signal inputted from thesecond input electric line (bus) 1011 into a signal beam emitted fromthe second light emitting portion 1015.

The first O/E converter 1007 includes a first light receiving portion1016. The second O/E converter 1008 includes a second light receivingportion 1017. The first light receiving portion 1016 is disposed in aposition whose position in the direction of the width is the same asthat of the second light emitting portion 1015 on the first surface 1002of the graded index slab waveguide 1001, and receives the second beam1019 (wavelength: X). The second light receiving portion 1017 receivesthe first beam 1018 having the same wavelength as the second beam 1019in a position whose position in the direction of the width is the sameas that of the first light emitting portion 1014 on the second surface1003 of the graded index slab waveguide 1001. The first O/E converter1007 is connected to the first output electric line (bus) 1012. Thefirst O/E converter 1007 converts the received signal beam into anexternal electric signal outputted to the outside by the first outputelectric line (bus) 1010. The second O/E converter 1008 is connected tothe second output electric line (bus) 1013. The second O/E converter1008 converts the received signal beam into an external electric signaloutputted to the outside by the second output electric line (bus) 1013.

The slab length L of the graded index slab waveguide 1001 substantiallycoincides with 8×n₀×W₀ ²/λ. Here, the effective refractive index of the0th-order mode beam excited in the direction of the width is n₀. Theslab length L of the graded index slab waveguide 1001 corresponds to thecase where a plurality of incident beams is superimposed one on anotherin (Expression 2) of (1) Asymmetrical incidence described in the firstembodiment. By setting the slab length L like this, the first beam 1018incident from the first light emitting portion 1014 forms an imagehaving the same profile as that when the beam is incident in thevicinity of the second light receiving portion 1017. Likewise, thesecond beam 1019 incident from the second light emitting portion 1015forms an image having the same profile as that when the beam is incidentin the vicinity of the first light receiving portion 1016. According tothe self-imaging principle, since a similar phenomenon occurs everylength of 8×n₀×W₀ ²/λ, by setting the slab length L to an integralmultiple of 8×n₀×W₀ ²/λ, the length of the graded index slab waveguide1001 can be adjusted according to use. The detailed mechanism ofsplitting and the mechanism in which there is no signal beam waveformdisturbance in the direction of the thickness and in the direction ofthe width even in the case of high-speed transmission are similar tothose of the first embodiment.

By the above structure, when an external electric signal is inputted tothe first E/O converter 1006 from the first input electric line 1010,the first E/O converter 1006 converts the external electric signal intothe first beam 1018 emitted from the first light emitting portion 1014.Moreover, when an external signal is inputted to the second E/Oconverter 1009 from the second input electric line 1011, the second E/Oconverter 1009 converts the external electric signal into the secondbeam 1019 emitted from the second light emitting portion 1015.

The first beam 1018 emitted from the first light emitting portion 1014is incident on the graded index slab waveguide 1001 through the incidentsurface 1002 to be transmitted. The first beam 1018 forms, according tothe self-imaging principle, an image having the same profile as thatwhen the beam is incident in the vicinity of the second light receivingportion 1017. By this, the first beam 1014 is outputted from the secondsurface 1003 to the second light receiving portion 1017. The secondlight receiving portion 1017 outputs an electric signal corresponding tothe received first beam 1018. The outputted electric signal is outputtedto the outside from the second output electric line 1013. On the otherhand, the second beam 1019 emitted from the second light emittingportion 1015 is incident on the graded index slab waveguide 1001 throughthe second surface 1003 to be transmitted. The second beam 1019 forms,according to the self-imaging principle, an image having the sameprofile as that when the beam is incident in the vicinity of the firstlight receiving portion 1016. By this, the second beam 1019 is outputtedfrom the first surface 1002 to the first light receiving portion 1016.The first light receiving portion 1016 outputs an electric signalcorresponding to the received second beam 1019. The first lightreceiving portion 1016 outputs an electric signal corresponding to thereceived second beam 1019. The outputted electric signals are outputtedto the outside from the first output electric line 1012. As describedabove, since the MMI is reversible irrespective of the beam transmissiondirection, the MMI can be used in both directions. Consequently, it isunnecessary to provide separate optical waveguides to straightlytransmit two signal beams in both directions, so that two signal beamscan be independently transmitted in both directions with one gradedindex slab waveguide 1001.

FIG. 11 is a perspective view showing the general outline of a four-pairtwo-way straight sheet bus which is an optical device according to amodification of the eighth embodiment of the present invention. Theschematic structure of the optical device of the modification is thesame as that of the previously-described single-pair two-way straightsheet bus. The optical device of the modification is provided with agraded index slab waveguide 1101, a first array E/O converter 1106, asecond array E/O converter 1109, a first array O/E converter 1107, asecond array O/E converter 1108, a first input electric line (bus) 1010,a second input electric line (bus) 1011, a first output electric line(bus) 1012 and a second output electric line (bus) 1013.

While the first array E/O converter 1106 has substantially the samestructure as the first E/O converter 1006 of the single-pair two-waystraight sheet bus, it is different in that a first light emittingportion group 1114 comprising four light emitting portions is formedinstead of the first light emitting portion 1014. While the second arrayE/O converter 1109 has substantially the same structure as the secondE/O converter 1009 of the single-pair two-way straight sheet bus, it isdifferent in that a second light emitting portion group 1115 comprisingfour light emitting portions is formed instead of the second lightemitting portion 1015. While the first array O/E converter 1107 hassubstantially the same structure as the first O/E converter 1007 of thesingle-pair two-way straight sheet bus, it is different in that a firstlight receiving portion group 1116 comprising four light receivingportions is formed instead of the first light receiving portion 1016.While the second array O/E converter 1108 has substantially the samestructure as the second O/E converter 1008 of the single-pair two-waystraight sheet bus, it is different in that a second light receivingportion group 1117 comprising four light receiving portions is formedinstead of the second light receiving portion 1017. The positions, inthe direction of the width, of the light emitting portions included inthe first light emitting portion group 1114 all correspond to those ofthe light receiving portions included in the second light receivingportion group 1117. The positions, in the direction of the width, of thelight emitting portions included in the second light emitting portiongroup 1115 all correspond to those of the light receiving portionsincluded in the first light receiving portion group 1116.

The first light emitting portion group 1114 makes a first beam 1121 to afourth beam 1124, which are four signal beams all having the samewavelength, independently incident on the graded index slab waveguide1121 through a first surface 1102 based on the external electric signalinputted from the first input electric line 1010. The graded index slabwaveguide 1101 transmits the first beam 1121 to the fourth beam 1124.The first beam 1121 to the fourth beam 1124 exit from a second surface1103 and are received by the light receiving portions, whose positionsin the direction of the width are the same, of the second lightreceiving portion group 1117 like in the case of the graded index slabwaveguide 1001. The received signals are outputted to the outside by thesecond output electric line 1013.

The second light emitting portion group 1115 makes a fifth beam 1125 toan eighth beam 1128, which are four signal beams all having the samewavelength, independently incident on the graded index slab waveguide1101 through the second surface 1103 based on the external electricsignal inputted from the second input electric line 1011. The gradedindex slab waveguide 1101 transmits the fifth beam 1125 to the eighthbeam 1128. The fifth beam 1125 to the eighth beam 1128 exit from thefirst surface 1102 and are received by the light receiving portions,whose positions in the direction of the width are the same, of the firstlight receiving portion group 1116 like in the case of the graded indexslab waveguide 1001. The received signals are outputted to the outsideby the first output electric line 1011.

The principle that four incident beams independently appear in parallelpositions in the direction of the width corresponds to the case where aplurality of incident beams is superimposed one on another in(Expression 2) of (1) Asymmetrical incidence described in the firstembodiment. As described above, by using the MMI, it is unnecessary toprovide separate optical waveguides to straightly transmit four pairs ofsignal beams in both directions, and four pair of signal beams can beindependently transmitted with one graded index slab waveguide 1101.

While the eighth embodiment shows examples of the single-pair two-waystraight sheet bus and the four-pair two-way straight sheet bus,generally, N×M-signal straight sheet bus (N, M=1, 2, 3, . . . ) can bedesigned in like manner. In this case, by making a number, N, ofincident beams incident on given positions on a first surface and makinga number, M, of incident beams incident from given positions on a secondsurface in a graded index slab waveguide having a slab length L which issubstantially an integral multiple of the following expression, anumber, N, of exiting beams can be obtained from positions, whosepositions in the direction of the width are the same, of the secondsurface and a number, M, of exiting beams can be obtained frompositions, whose positions in the direction of the width are the same,of the first surface. $\frac{8n_{0}W_{0}^{2}}{\lambda}$

Ninth Embodiment

FIG. 12 is a perspective view showing the general outline of asingle-pair two-way cross sheet bus which is an optical device of aninth embodiment of the present invention. In the optical deviceaccording to the ninth embodiment, one signal beam transmissiondirection of the two-signal cross sheet bus described in the fourthembodiment is reversed to enable two-way communication. The opticaldevice according to the ninth embodiment comprises as a main element agraded index slab waveguide 1201. The graded index slab waveguide 1201is, as shown in FIG. 12, a sheet-form multi-mode optical transmissionline that extends parallel to the x-z plane. The graded index slabwaveguide 1201 has a distribution such that the highest refractive indexn_(max) is provided at the center in the direction of the thickness andthe refractive index does not increase with distance from the center.The graded index slab waveguide 1201 has a uniform refractive index inthe direction of the width and has no refractive index distribution. Theoptical device according to the ninth embodiment is provided with afirst E/O converter 1206, a second E/O converter 1209, a first O/Econverter 1207, a second O/E converter 1208, a first input electric line(bus) 1010, a second input electric line (bus) 1011, a first outputelectric line (bus) 1012 and a second output electric line (bus) 1013.

The first E/O converter 1206 includes a first light emitting portion1214. The second E/O converter 1209 includes a second light emittingportion 1215. The first light emitting portion 1214 makes a first beam1218 (wavelength: λ) incident on a given position in the direction ofthe width on a first surface 1202 of the graded index slab waveguide1201. The second light emitting portion 1215 makes a second beam 1219having the same wavelength as the first beam 1218 incident on a givenposition in the direction of the width on a second surface 1203 of thegraded index slab waveguide 1201. The first E/O converter 1206 isconnected to the first input electric line (bus) 1010. The first E/Oconverter 1206 converts an external electric signal inputted to thefirst input electric line (bus) 1010 into a signal beam emitted from thefirst light emitting portion 1214. The second E/O converter 1209 isconnected to the second input electric line (bus) 1013. The second E/Oconverter 1209 converts an external electric signal inputted from thesecond input electric line (bus) 1013 into a signal beam emitted fromthe second light emitting portion 1215.

The first O/E converter 1207 includes a first light receiving portion1216. The second O/E converter 1208 includes a second light receivingportion 1217. The first light receiving portion 1216 is disposed in aposition whose position in the direction of the width is symmetrical tothe second light emitting portion 1015 with respect to the center on thefirst surface 1202 of the graded index slab waveguide 1201, and receivesthe second beam 1219 (wavelength: λ). The second light receiving portion1217 receives the first beam 1218 having the same wavelength as thesecond beam 1219 in a position whose position in the direction of thewidth is symmetrical to the first light emitting portion 1214 withrespect to the center on the second surface 1203 of the graded indexslab waveguide 1201. The first O/E converter 1207 is connected to thefirst output electric line (bus) 1012. The first O/E converter 1207converts the received signal beam into an external electric signaloutputted to the outside by the first output electric line (bus) 1012.The second O/E converter 1208 is connected to the second output electricline (bus) 1011. The second O/E converter 1208 converts the receivedsignal beam into an external electric signal outputted to the outside bythe second output electric line (bus) 1011.

The slab length L of the graded index slab waveguide 1201 substantiallycoincides with 4×n₀×W₀ ²/λ. Here, the effective refractive index of the0th-order mode beam excited in the direction of the width is n₀. Theslab length L of the graded index slab waveguide 1201 corresponds to thecase where a plurality of incident beams is superimposed one on anotherin (Expression 3) of (1) Asymmetrical incidence described in the firstembodiment. By setting the slab length L like this, the first beam 1218incident from the first light emitting portion 1214 forms an imagehaving the same profile as that when the beam is incident in thevicinity of the second light receiving portion 1217. Likewise, thesecond beam 1219 incident from the second light emitting portion 1215forms an image having the same profile as that when the beam is incidentin the vicinity of the first light receiving portion 1216. According tothe self-imaging principle, since a similar phenomenon occurs everylength of 8×n₀×W₀ ²/λ, by setting the slab length L to an integralmultiple of 4×n₀×W₀ ²/λ the length of the graded index slab waveguide1201 can be adjusted according to use. The detailed mechanism ofsplitting and the mechanism in which there is no signal beam waveformdisturbance in the direction of the thickness and in the direction ofthe width even in the case of high-speed transmission are similar tothose of the first embodiment.

By the above structure, when an external electric signal is inputted tothe first E/O converter 1206 from the first input electric line 1010,the first E/O converter 1206 converts the external electric signal intothe first beam 1218 emitted from the first light emitting portion 1214.Moreover, when an external signal is inputted to the second E/Oconverter 1209 from the second input electric line 1013, the second E/Oconverter 1209 converts the external electric signal into the secondbeam 1219 emitted from the second light emitting portion 1215.

The first beam 1218 emitted from the first light emitting portion 1214is incident on the graded index slab waveguide 1201 through the firstsurface 1202 to be transmitted. The first beam 1218 forms, according tothe self-imaging principle, an image having the same profile as thatwhen the beam is incident in the vicinity of the second light receivingportion 1217. By this, the first beam 1218 is outputted from the secondsurface 1203 to the second light receiving portion 1217, and isoutputted to the outside from the second output electric line 1011. Onthe other hand, the second beam 1219 emitted from the second lightemitting portion 1215 is incident on the graded index slab waveguide1201 through the second surface 1203 to be transmitted. The second beam1219 forms, according to the self-imaging principle, an image having thesame profile as that when the beam is incident in the vicinity of thefirst light receiving portion 1216. By this, the second beam 1219 isoutputted from the first surface 1202 to the first light receivingportion 1216. The first light receiving portion 1216 outputs an electricsignal corresponding to the received second beam 1219. The first lightreceiving portion 1216 outputs an electric signal corresponding to thereceived second beam 1219. The outputted electric signals are outputtedto the outside from the first output electric line 1012. As describedabove, since the MMI is reversible irrespective of the beam transmissiondirection, the MMI can be used in both directions. Consequently, it isunnecessary to provide separate optical waveguides to transmit twosignal beams so as to cross each other in both directions, so that twosignal beams can be independently transmitted in both directions withone graded index slab waveguide 1201.

While the ninth embodiment shows an example of the single-pair two-waystraight sheet bus, generally, N×M-signal cross sheet bus (N, M=1, 2, 3,. . . ) can be designed in like manner. In this case, by making anumber, N, of incident beams incident on given positions on a firstsurface and making a number, M, of incident beams incident from givenpositions on a second surface in a graded index slab waveguide having aslab length L which is substantially an odd multiple of the followingexpression, a number, N, of exiting beams can be obtained frompositions, symmetrical with respect to the center in the direction ofthe width, of the second surface and a number, M, of exiting beams canbe obtained from positions, symmetrical with respect to the center inthe direction of the width, of the first surface.$\frac{4n_{0}W_{0}^{2}}{\lambda}$

Tenth Embodiment

FIG. 13 is a schematic diagram of the structure of a single-pair two-waystraight sheet bus array which is an optical integrated device accordingto a tenth embodiment of the present invention. The optical integrateddevice of the tenth embodiment comprises as a main element a laminatedtwo-way straight sheet bus 1301 in which eight single-pair two-waystraight sheet buses of the eighth embodiment are laminated in thedirection of the thickness as shown in FIG. 13. The optical integrateddevice of the tenth embodiment is provided with the laminated two-waystraight sheet bus 1301, a first array E/O converter 1302, a first arrayO/E converter 1303, a second array E/O converter 1305, a second arrayO/E converter 1304, a first input electric line (bus) 1306, a secondinput electric line (bus) 1308, a first output electric line (bus) 1307and a second output electric line (bus) 1309.

In the first array E/O converter 1302, eight light emitting portionsdisposed on one end surface (left side of the figure) of the two-waystraight sheet buses are formed into an array. In the first array O/Econverter 1303, eight light receiving portions disposed on one endsurface (left side of the figure) of the two-way straight sheet busesare formed into an array. The first array E/O converter 1302 and thefirst array O/E converter 1303 are disposed so as to adjoin each other.In the second array E/O converter 1305, eight light emitting portionsdisposed on the other end surface (right side of the figure) of thetwo-way straight sheet buses are formed into an array. In the secondarray O/E converter 1304, eight light receiving portions disposed on oneend surface (right and left side of the figure) of the two-way straightsheet buses are formed into an array. The second array E/O converter1305 and the second array O/E converter 1304 are disposed so as toadjoin each other. The light emitting portions of the first array E/Oconverter 1302 and the light receiving portions of the second array O/Econverter 1304 are disposed so as to be opposed to each other with thetwo-way straight sheet buses in between. The light emitting portions ofthe second array E/O converter 1305 and the light receiving portions ofthe second array O/E converter 1303 are disposed so as to be opposed toeach other with the two-way straight sheet buses in between. The firstarray E/O converter 1302 is connected to the first input electric line1306. The second array E/O converter 1305 is connected to the secondinput electric line 1308. The first array O/E converter 1303 isconnected to the first output electric line 1307. The second array O/Econverter 1304 is connected to the second output electric line 1309.

In the above structure, electric signals inputted from the first inputelectric line 1306 are converted into signal beams by the first arrayE/O converter 1302. The converted signal beams are emitted from thelight emitting portions and incident on the two-way straight sheet busesas incident beams. On the other hand, electric signals inputted from thesecond input electric line 1308 are converted into signal beams by thesecond array E/O converter 1305. The converted signal beams are emittedfrom the light emitting portions and incident on the two-way straightsheet buses as incident beams. The signal beams are transmitted andform, according to the self-imaging principle, images having the sameprofiles as the incident beams in the vicinity of the light receivingportions. The images are made to exit to the light receiving portions ofthe two-way straight sheet buses. The first array O/E converter 1303having received the exiting beams by the light receiving portionsconverts the exiting beams into electric signals and outputs them to thefirst output electric line 1307. The second array O/E converter 1304having received the exiting beams by the light receiving portionsconverts the exiting beams into electric signals and outputs them to thesecond output electric line 1309. In this manner, two-way straight sheetbuses are realized with a simple structure.

While the tenth embodiment discloses a laminated two-way straight sheetbus in which the included straight sheet buses are all single-pairtwo-way straight sheet buses, the present invention is not limitedthereto. For example, the single-pair two-way cross sheet buses of theninth embodiment may be laminated. Moreover, the optical integrateddevice of the tenth embodiment may be a two-way optical bus array inwhich two kinds of the single-pair straight sheet bus according to theeighth embodiment and the single-pair two-way cross sheet bus accordingto the ninth embodiment are laminated, or may be a composite optical busarray in which a plurality of kinds of optical buses selected from amongthe two-way sheet buses according to the eighth embodiment and the ninthembodiment and the optical sheet bus according to the third embodimentand the fourth embodiment are laminated.

Eleventh Embodiment

FIG. 14 is a schematic diagram of the structure of a multi-layer opticalbus which is an optical integrated device according to an eleventhembodiment of the present invention. In the optical integrated deviceaccording to the eleventh embodiment, graded index slab waveguides whosesizes in the direction of the length are different are laminated in thedirection of the thickness. As the graded index slab waveguides to belaminated, any of the single-layer graded index slab waveguides such asthe ones of the first to fifth, eighth and ninth embodiments may beused. According to the self-imaging principle of the multi-modeinterference, since a similar effect occurs every predetermined period,graded index slab waveguides having different lengths may be used evenin the case of the same kind of optical devices. For example, in theoptical integrated device described in FIG. 14, straight sheet busesusing graded index slab waveguides are laminated. In the opticalintegrated device, the incident surfaces are aligned, and the lengths,in the direction of the width, of a first graded index slab waveguide1401, a second graded index slab waveguide 1402 and a third graded indexslab waveguide 1403 are L₁=8 n₀W₀ ²/λ, L₂=16 n₀W₀ ²/λ and L₁=32 n₀W₀²/λ, respectively. By structuring the optical integrated device likethis, the profiles of the exiting beams from the graded index slabwaveguides whose sizes in the direction of the length are different canbe made the same.

In actuality, the distances between apparatuses, substrates or chipsmounted on a substrate are not the same. Therefore, in actuality,optical sheet buses corresponding to the various lengths are necessary.By laminating optical sheet buses of different lengths into one devicelike the eleventh embodiment, optical transmission from a substrate to aplurality of substrates at different distances is enabled. At this time,an end surface 140 comprising the aligned end surfaces of themulti-layer optical bus is connected to the substrate, and as the inputand output to the side where the end surfaces are aligned, for example,a 1×N optical splitter (an N×1 optical combiner) according to the firstembodiment may be used.

Twelfth Embodiment

FIG. 21A is a perspective view showing the general outline of a gradedindex slab waveguide 2101 of an optical device that performs beamdemultiplexing according to a twelfth embodiment of the presentinvention. FIG. 21B is a cross-sectional view of the graded index slabwaveguide 2101 of the optical device that performs beam demultiplexingaccording to the twelfth embodiment of the present invention. FIG. 22Ais a result of a BPM (beam propagation method) simulation performed whena signal beam of 1.30 μm is transmitted through the graded index slabwaveguide 2101. FIG. 22B is a result of a BPM simulation performed whena signal beam of 1.55 μm is transmitted through the graded index slabwaveguide 2101.

The optical device according to the twelfth embodiment comprises as amain element the graded index slab waveguide 2101 that transmits beams.The graded index slab waveguide 2101 is, as shown in FIG. 21A, asheet-form multi-mode optical transmission line that extends parallel tothe x-z plane. The graded index slab waveguide 2101 has a refractiveindex distribution in the direction of the thickness such that thehighest refractive index n_(max) is provided at the central position inthe direction of the thickness and the refractive index does notincrease with distance from the center. The graded index slab waveguide2101 has a uniform refractive index in the direction of the width andhas no refractive index distribution. The graded index slab waveguide2101 includes an incident surface 2102 and an exit surface 2103.

The incident surface 2102 is opposed to an incident portion (not shown)that makes a multiplex incident beam 2107 comprising multiplexed beamsof two different wavelengths (a wavelength 1.30 μm and a wavelength 1.55μm) incident on a position a predetermined distance away from the centerin the direction of the width. The exit surface 2103 is opposed to anexit portion (not shown) that receives two exiting beams 2108 and 2109of different wavelengths that exit from positions symmetrical to eachother with respect to the center in the direction of the width. Theincident portion makes the multiplex incident beam 2107 incident on aposition a predetermined distance away from the center, in the directionof the width, of the incident surface 2102. The multiplex incident beam2107 is transmitted inside the graded index slab waveguide 101. Insidethe graded index slab waveguide 2101, the multiplex incident beam 2107is demultiplexed into two beams in accordance with the wavelengthaccording to the self-imaging principle of the multi-mode interferencedescribed later, exits as the two exiting beams 2108 and 2109 havingdifferent wavelengths (a wavelength 1.30 μm and a wavelength 1.55 μm)from positions away from each other in the direction of the width of theexit surface 2103, and reaches the exit portion. The slab length L ofthe graded index slab waveguide 2101 is an optical path length where thephase difference between the light quantity movement of the wavelength1.30 μm and the light quantity movement of the wavelength 1.55 μm isopposite phase (that is, an integral multiple of π).

The refractive index distribution, in the direction of the thickness, ofthe graded index slab waveguide 2101 is expressed, for example, by themaximum point n_(max) of the refractive index which point is situated atthe center in the direction of the thickness, the distance r from themaximum point in the direction of the thickness and the refractive indexdistribution constant A^(1/2) as shown by the previously-mentioned(Expression 1).

The refractive index distribution constant is optimized according to thefilm thickness of the graded index slab waveguide 2101 and the profileof the multiplex incident beam 2107 so that the beam transmitted in thegraded index slab waveguide 2101 does not spread outside the filmthickness. For example, when the spread angle of the multiplex incidentbeam 2107 is large compared to the film thickness of the graded indexslab waveguide 2101, the refractive index distribution constant isincreased. Conversely, when the spread angle of the multiplex incidentbeam 2107 is small, the refractive index distribution constant isdecreased. Moreover, by adjusting the film thickness of the graded indexslab waveguide 2101 in accordance with the beam diameter of themultiplex incident beam 2107, the coupling loss can be reduced. Therefractive index distribution is not necessarily a continuous change asshown in (Expression 1); it may stepwisely change as a function of thedistance from the center.

Next, a mechanism will be described of, when the multiplex incident beam2107 incident on a position a predetermined distance away from thecenter in the direction of the width is incident on the incident surface2102 of the graded index slab waveguide 2101, demultiplexing theincident beam in to two beams in accordance with the wavelengthsymmetrically with respect to the central line in the direction of thewidth on the side of the exit surface 2103. Description will beseparately given for (i) the case of a beam transmitted within thecentral plane in the direction of the thickness (signal beam transmittedon the optical path designated A in FIG. 21B; and (ii) a beam nottransmitted within the central plane in the direction of the thickness.As the beam of (ii) not transmitted within the central plane in thedirection of the thickness, the following two signal beams are present:the case of an incident beam that is incident with an axis shift angleon the central plane (signal beam transmitted on the optical pathdesignated B in FIG. 21B) and the case of an incident beam that isincident on a position position-shifted (axis-shifted) from the centralplane (signal beam transmitted on the optical path designated C in FIG.21B). The beam of (i) transmitted within the central plane in thedirection of the thickness is not affected by the refractive indexdistribution in the direction of the thickness. On the other hand, thebeam of (ii) not transmitted within the central plane in the directionof the thickness is affected by the refractive index distribution in thedirection of the thickness.

In the graded index slab waveguide 2101, the behavior of the beam of (i)transmitted within the central plane in the direction of the thicknesswhich behavior is affected substantially only by the effectiverefractive index no is equivalent to that in a case where the uniformrefractive index is the effective refractive index n₀ in the slabwaveguide described in Document (11). Therefore, the condition of theexiting beams with respect to the multiplex incident beam 2107transmitted within the central plane, in the direction of the thickness,of the graded index slab waveguide 2101 varies according to the slablength L by the multi-mode mode dispersion excited in the direction ofthe width of the slab waveguide whose refractive index is no anduniform. Here, that the condition of the exiting beams varies means thatthe number and exit positions of images the same as the incident beamvary. In the case of the graded index slab waveguide 2101 according tothe twelfth embodiment, by the slab length L being the optical pathlength where the phase difference between the light quantity movement ofthe wavelength 1.30 μm and the light quantity movement of the wavelength1.55 μm is opposite phase (that is, an integral multiple of π), twoimages the same as the multiplex incident beam 2107 having differentwavelengths are formed on the exit surface 2103 so as to be symmetricalwith respect to the center in the direction of the width.

The BPM simulation of FIG. 22A shows the behavior of the beam of thewavelength 1.30 μm. Of the multiplex incident beam 2107, the signal beamcomponent corresponding to the wavelength 1.30 μm is developed into the0th-order mode (basic mode) intrinsic to the graded index slab waveguide2101 and the primary mode. The mode dispersion is different between the0th-order mode and the primary mode. In other words, the propagationconstant of the 0th-order mode and the propagation constant of theprimary mode are different from each other. Therefore, a modeinterference occurs between the 0th-order mode and the primary mode. Bythis mode interference, as shown in FIG. 22A, the signal beam componentcorresponding to the wavelength 1.30 μm is transmitted in one directionin the direction of the length (the direction from the left to the rightin the figure) while alternately moving in the direction of the width inthe graded index slab waveguide 2101.

On the other hand, the BPM simulation of FIG. 22B shows the behavior ofthe beam of the wavelength 1.55 μm. Of the multiplex incident beam 2107,the signal beam component corresponding to the wavelength 1.55 μm isalso developed into the 0th-order mode (basic mode) intrinsic to thegraded index slab waveguide 2101 and the primary mode. Therefore, a modeinterference occurs between the 0th-order mode and the primary mode likethe case of the signal beam component of the wavelength 1.30 μm. By thismode interference, as shown in FIG. 22B, the signal beam componentcorresponding to the wavelength 1.55 μm is transmitted in one directionin the direction of the length (the direction from the left to the rightin the figure) while alternately moving in the direction of the width inthe graded index slab waveguide 2101.

The signal beam component of the wavelength 1.30 μm is different fromthe signal beam component of the wavelength 1.55 μm in signal beamcomponent and wavelength dispersion. That is, since the signal beamcomponent of the wavelength 1.30 μm and the signal beam component of thewavelength 1.55 μm are different from each other in the propagationconstant of each mode, these beams exhibit different behaviors whentransmitted through the graded index slab waveguide 2101. Using thischaracteristic, in the optical device according to the twelfthembodiment, the slab length L of the graded index slab waveguide 2101 isset to a value where the phase difference between the light quantitymovement of the signal beam of the wavelength 1.30 μm and the lightquantity movement of the signal beam of the wavelength 1.55 μm isopposite phase (that is, an integral multiple of π). By structuring theoptical device like this, according to the self-imaging principle of themulti-mode interference, an image having the same profile as theincident beam of the signal component of the wavelength 1.30 μm and animage having the same profile as the incident beam of the signalcomponent of the wavelength 1.55 μm can be formed in differentpositions. Then, the two images formed in different positions are madeto exit as the exiting beam 2108 and the exiting beam 2109,respectively, whereby beam demultiplexing is achieved.

On the other hand, the beam of (ii) not transmitted within the centralplane in the direction of the thickness propagates along the centralplane while meandering in the direction of the thickness as shown inFIG. 21B, because it is affected by the refractive index distribution inthe direction of the thickness. That is, since the beam traveling in adirection away from the central plane always travels from a part wherethe refractive index is relatively high to a part where the refractiveindex is relatively low, as the beam travels, the angle between thedirection of travel and the direction of the thickness graduallyincreases, and becomes 90° at the position farthest from the centralaxis. Moreover, since the beam traveling in a direction toward thecentral plane always travels from a part where the refractive index isrelatively low to a part where the refractive index is relatively high,as the beam travels, the angle between the direction of travel and thedirection of the thickness gradually decreases, and becomes smallest atthe position intersecting the central plane. Since the refractive indexthat affects the beam of (ii) not transmitted within the central planein the direction of the thickness is always lower than the refractiveindex no although it makes the beam meander, the speed of the beam of(ii) is higher than that of the beam of (i) transmitted within thecentral axis in the direction of the thickness.

When the refractive index distribution is the refractive indexdistribution of the quadratic function shown in the previously-shown(Expression 1), the component of the transmission speed, parallel to thecentral plane, of the beam of (ii) not transmitted within the centralplane in the direction of the thickness is equal to the transmissionspeed of the beam of (i) transmitted within the central plane in thedirection of the thickness. This means that there is no mode dispersionin the direction of the thickness. Therefore, the component, parallel tothe central plane of the beam of (ii) not transmitted within the centralplane in the direction of the thickness (component, in a directionvertical to the direction of the thickness, of a meandering beam) of theincident beam is demultiplexed into two beams symmetrically with respectto the center in the direction of the width at the exit surface like thebeam of (i) transmitted within the central plane in the direction of thethickness.

Since the component, vertical to the central plane of the beam of (ii)not transmitted within the central plane in the direction of thethickness (component in the direction of the thickness of a meanderingbeam) of the incident beam changes according to the propagation positionof the meandering beam, the condition of the exiting beam cannot bedetermined. However, the component in the direction of the thickness ofthe meandering beam is not affected by a signal waveform disturbance dueto the mode dispersion, because the mode dispersion in the direction ofthe thickness does not occur. For this reason, the component behavesequivalently to that in the case where there is no influence of the modedispersion also in the direction of the width. Therefore, the twoexiting beams have the same images as the signal beam componentscorresponding to the wavelengths of the multiplex incident beam. Fromthe above result, the beam of (ii) not transmitted within the centralplane in the direction of the thickness (meandering beam) isdemultiplexed into two beams as the same image as the incident beamsymmetrically with respect to the center in the direction of the widthaccording to the slab waveguide configuration like in the case of (i).

As described above, since the incident beam is equally demultiplexedinto two beams with respect to all the eigenmodes in the direction ofthe thickness of the graded index slab waveguide 2101, an optical devicecan be obtained that functions, if the multiplex incident beam isincident on a position a predetermined distance away from the center, inthe direction of the width, of the incident surface, as a devicedemultiplexing the incident beam into two beams even when the incidentbeam is position-shifted from the center in the direction of thethickness or has a large spread angle. Since the position shift, fromthe center in the direction of the width, of the incident beam is acause of an imbalance in the demultiplexing ratio between the exitingbeams, when it is intended to obtain equal exiting beams, it ispreferable that the position shift be minimized. However, it is possibleto adjust the intensity ratio between the signal beams by actively usingthe position shift.

When the position where opposite phase occurs between the twowavelengths is determined, either of the following methods may beadopted: determining the position from the position, in the direction ofthe width, where the ratio between the light quantities of the twoexiting beams is highest; and determining the position from theposition, in the direction of the width, where the light quantities ofthe two exiting beams are smallest. When the former method is adopted,the exiting beam loss can be reduced, and the efficiency of use of thetransmitted signal beams can be improved. When the latter method isadopted, since the error component included in the exiting beams can bereduced, the transmission error can be reduced.

While the first to twelfth embodiments show examples of verticalincidence and exit on and from the end surfaces as the method of inputand output to and from the graded index slab waveguide, the presentinvention is not limited thereto. FIG. 15A is a perspective view showingan example of the incidence and exit method of the graded index slabwaveguide. FIG. 15B is a perspective view showing another example of theincidence and exit method of the graded index slab waveguide. FIG. 16 isa perspective view showing still another example of the incident andexit method of the graded index slab waveguide. For example, areflection method (FIG. 15A) may be adopted in which a reflectingsurface 1502 and a reflecting surface 1503 are formed by inclining theincident and exit end surfaces of the graded index slab waveguide 450and the incident and exiting beams incident from a direction vertical tothe direction of the thickness are reflected by the reflecting surface1502 and the reflecting surface 1503 to thereby bend the optical path900. Moreover, a coupler method (FIG. 16) may be adopted in which aprism 1602 is disposed in the vicinity of the incident and exiting endsurfaces of the graded index slab waveguide so as to be adjacent to thesurface in the direction of the thickness and beams incident and exitingon and from the prism 1602 are coupled to the optical bus. Moreover, adiffraction optical element such as a diffraction grating may be usedinstead of the prism 1602. Moreover, when the substrate used is anelectric-optical hybrid substrate formed by sandwiching a graded indexslab waveguide between the electric substrates 1503, as shown in FIG.15B, a graded index slab waveguide where reflecting surfaces are formedby inclining the incident and exit end surfaces 45° is used, and throughholes 1504 that pass a vertical incident beam therethrough are formed inparts of the electric substrate that are made to be reflecting surfacesby being inclined 45°.

Moreover, while the sheet-form graded index slab waveguide is on asingle plane in the first to twelfth embodiments, the present inventionis not limited thereto. FIG. 17A is a perspective view showing anexample of the configuration of the graded index slab waveguide, andFIG. 17B is a perspective view showing another example of theconfiguration of the graded index slab waveguide. As shown in FIG. 17A,a graded index slab waveguide 1701 may be curved (FIG. 17A) so that thecentral position in the direction of the thickness always draws the samecurve on given two different cross sections including the direction ofthe length and the direction of the thickness. Moreover, a graded indexslab waveguide 1702 may be twisted (FIG. 17B) so that the centralposition in the direction of the thickness draws different curves ongiven two different cross sections including the direction of the lengthand the direction of the thickness. This is because while in typicalslab waveguides where the refractive index is uniform in the directionof the thickness, the influence of the dispersion and the loss due to achange in incident angle when the beam is reflected at the interface bythe curve or the twist cannot be avoided, in the case of a slab having arefractive index having the maximum value at the center in the directionof the thickness, the beam does not reach the interface of the slab andpropagates irrespective of the condition of the interface of the slab.

While the input and exiting beams are limited to multi-mode beams in theabove description, the use of single-mode beams is not a problem whenthe coupling loss is not a problem.

Thirteenth Embodiment

FIG. 23 is a schematic diagram of the structure of an optical devicewhich is a 1×2 optical splitter according to a thirteenth embodiment ofthe present invention. As shown in FIG. 23, the optical device accordingto the thirteenth embodiment comprises as main elements a first gradedindex slab waveguide 2301, a second graded index slab waveguide 2302 anda third graded index slab waveguide 2303.

The first to third graded index slab waveguides 2301 to 2303 are each asheet-form multi-mode optical transmission line that extends parallel tothe x-z plane. The first to third graded index slab waveguides 2301 to2303 have a refractive index distribution in the direction of thethickness such that the highest refractive index n_(max) is provided atthe central position in the direction of the thickness and therefractive index does not increase with distance from the center. Thefirst to third graded index slab waveguides 2301 to 2303 have a uniformrefractive index in the direction of the width and have no refractiveindex distribution.

The first graded index slab waveguide 2301 is the same as the gradedindex slab waveguide described in the case of the splitting into twobeams according to the fist embodiment. That is, the slab length L₁ ofthe first graded index slab waveguide 2301 is a function of the basicmode width W₀ in the direction of the width, the effective refractiveindex no of the 0th-order mode beam excited in the direction of thewidth and the wavelength λ of the incident beam, and is approximatelyn₀×W₀ ²/(2λ).

The second graded index slab waveguide 2302 and the third graded indexslab waveguide 2302 are the same as the cross sheet bus according to thefourth embodiment. That is, the slab lengths L₂ of the second gradedindex slab waveguide 2301 and the third graded index slab waveguide 2303are a function of the basic mode width W₀ in the direction of the width,the effective refractive index n₀ of the 0th-order mode beam excited inthe direction of the width and the wavelength λ of the incident beam,and are both approximately 4×n₀×W₀ ²/λ.

The optical device according to the thirteenth embodiment makes anincident beam incident on a central position, in the direction of thewidth, of the incident surface of the first graded index slab waveguide2301 from a non-illustrated incident portion. The first graded indexslab waveguide 2301 transmits the incident beam that is incident on thecenter, in the direction of the width, of the incident surface, andgenerates two exiting beams that are symmetrical with respect to thecenter, in the direction of the width, of the exit surface according tothe self-imaging principle.

Of the exiting beams having exited from the first graded index slabwaveguide 2301, one exiting beam is incident on a position apredetermined distance away from the center, in the direction of thewidth, of the incident surface of the second graded index slab waveguide2302. The second graded index slab waveguide 2302 transmits the incidentbeam that is incident, and generates an exiting beam in a position, ofthe exit surface, that is symmetrical to the incident beam with respectto the center in the direction of the width according to theself-imaging principle.

Of the exiting beams having exited from the first graded index slabwaveguide 2301, the other exiting beam is incident on a position apredetermined distance away from the center, in the direction of thewidth, of the incident surface of the third graded index slab waveguide2303. The third graded index slab waveguide 2303 transmits the incidentbeam that is incident, and generates an exiting beam in a position, ofthe exit surface, that is symmetrical to the incident beam with respectto the center in the direction of the width according to theself-imaging principle.

Here, the width of the basic mode of the first graded index slabwaveguide 2301 is W₀, the predetermined distance from the center in thedirection of the width to the incident position of the incident beam onthe incident surface of the second graded index slab waveguide 2302 isx1, and the predetermined distance from the center in the direction ofthe width to the incident position of the incident beam on the incidentsurface of the third graded index slab waveguide 2303 is x2. In thiscase, the separation width d1 on the exit surface of the first gradedindex slab waveguide 2301 is equal to d1=W₀/2 according to theself-imaging principle. However, the separation width d2 between theexiting beams having exited from the second graded index slab waveguide2302 and the third graded index slab waveguide 2303 is equal tod2=W₀/2+2×x1+2×x2 and is largely increased.

As described above, in the optical device according to the thirteenthembodiment, the separation width can be increased without the basic modewidth W₀ being changed. For example, when the splitting into two beamsis structured only with the first graded index slab waveguide 2301, theseparation width after the beam is split into two beams is W₀/2, andwhen an optical fiber with a diameter of 125 μm is connected, it isnecessary that the basic mode width W₀ be not less than 250 μm. In thiscase, the length of the first graded index slab waveguide 2301 is notless than L=35,000 μm, and size increase cannot be avoided. Further,when a plastic optical fiber with a diameter of 200 to 1.000 μm isconnected, L>100.000 μm (L is proportional to the square of W₀).

On the other hand, when the second graded index slab waveguide 2302 andthe third graded index slab waveguide 2303 are used, the movementamounts of the two exiting beams are 2×x1+2×x2. The values of x1 and x2can be increased to the half breadth of the second graded index slabwaveguide 2302 and the third graded index slab waveguide 2303.Therefore, the slab length L can be made smaller than that when it isintended to obtain an equal separation width by using only the firstgraded index slab waveguide 2301. As described above, even when thefirst graded index slab waveguide 2301 is an optical splitter with asmall separation width, the separation width can be easily increased byconnecting the second and third graded index slab waveguides.

FIG. 24 is a top view showing a relevant part of an optical device forincreasing the distance between three or more signal beams according toa first modification of the thirteenth embodiment of the presentinvention. The optical device according to the first modification of thethirteenth embodiment comprises, as shown in FIG. 24, a plurality ofgraded index slab waveguides disposed in the direction of the width. Thegraded index slab waveguides are each the same as the graded index slabwaveguide described in the case of the cross sheet bus according to thefourth embodiment.

The optical device according to the first modification of the thirteenthembodiment comprises as main elements, a (k−1)-th graded index slabwaveguide 2401 which is the (k−1)-th graded index slab waveguide fromthe top of the figure in the direction of the width, a k-th graded indexslab waveguide 2402 which is the k-th graded index slab waveguide fromthe top of the figure in the direction of the width and a (k+1)-thgraded index slab waveguide 2403 which is the (k+1)-th graded index slabwaveguide from the top of the figure in the direction of the width.

In the optical device according to the first modification of thethirteenth embodiment, to increase the distance between three or moresignal beams, transmission is performed by appropriately combining thefollowing two patterns:

-   -   (1) A method in which the relationship between the incident        position and the center in the direction of the width is made        different directions like the adjoining k-th and (k+1)-th graded        index slab waveguides. In this case, the distance between the        signal beams can be largely increased.

(2) A method in which the relationship between the incident position andthe center in the direction of the width is made the same direction sothat the distance between the incident position and the center, in thedirection of the width, of the k-th graded index slab waveguide havingthe incident position closer to the center of the overall optical deviceis made smaller than the distance between the incident position and thecenter, in the direction of the width, of the (k−1)-th graded index slabwaveguide like the adjoining k-th and (k−1)-th graded index slabwaveguides.

By appropriately combining the methods described in (1) and (2), thedistance between three or more signal beams can be increased.

FIG. 25 is a top view showing a relevant part of an optical device forincreasing the distance between signal beams according to a secondmodification of the thirteenth embodiment of the present invention. Asshown in FIG. 24, the optical device according to the secondmodification of the thirteenth embodiment is provided with a pluralityof graded index slab waveguides disposed so that the incident positionsare shifted from one another in the direction of the length. The gradedindex slab waveguides are each the same as the graded index slabwaveguide described in the case of the cross sheet bus according to thefourth embodiment.

In the optical device according to the second modification of thethirteenth embodiment, the graded index slab waveguides are connected inmultiple stages in the direction of the length. That is, the exitingbeam from a graded index slab waveguide 2501 of the first stage isconnected as the incident beam of a graded index slab waveguide 2502 ofthe second stage, and the exiting beam from the graded index slabwaveguide 2502 of the second stage is successively connected as theincident beam of a graded index slab waveguide 2503 of the n-th stage.At this time, the optical device of the second modification of thethirteenth embodiment is arranged so that the positions of exit from thegradient index slab waveguides are always shifted in the same directionfrom the center in the direction of the width. By this arrangement, theexiting beam can be moved in the direction of the width.

While in all of the above-described optical devices, square graded indexslab waveguides that are independent of each other are connected tothereby increase the separation width, the present invention is notlimited thereto. For example, a graded index slab waveguide may bemanufactured that has a condition where a plurality of sheet-formmulti-mode waveguides is connected.

Fourteenth Embodiment

FIG. 26 is a perspective view showing the general outline of thestructure of an optical device having a beam converter according to afourteenth embodiment of the present invention. The optical deviceaccording to the fourteenth embodiment is provided with a graded indexslab waveguide 2610, an incident side optical fiber 2620, a first exitside optical fiber 2630, a second exit side optical fiber 2635, and anincident side beam converter 2640, a first exit side beam converter 2650and a second exiting beam converter 2655.

The graded index slab waveguide 2610 has the same structure as thegraded index slab waveguide 101 constituting the optical device of thefirst embodiment, and has a predetermined slab length L performing thesplitting into two beams. The incident side optical fiber 2620, thefirst exit side optical fiber 2630 and the second exit side opticalfiber 2635 are all GI (graded index) multi-mode optical fibers. Theincident side optical fiber 2620, the first exit side optical fiber 2630and the second exit side optical fiber 2635 all have a refractive indexdistribution such that the highest refractive index is provided at thecenter and the refractive index decreases toward the peripherysubstantially along a quadratic function.

The incident side beam converter 2640 is disposed between the incidentsurface 2612 of the graded index slab waveguide 2610 and the exit sideend surface 2622 of the incident side optical fiber 2620. The incidentside beam converter 2640 is substantially cylindrical, and has arefractive index distribution such that the refractive index is themaximum at the central axis of the cylinder and decreases toward theperiphery. The refractive index distribution of the incident side beamconverter 2640 is such that the refractive index is the maximum at thecenter and changes toward the periphery substantially along thequadratic function.

The incident side beam converter 2640 has a refractive indexdistribution such that the change gradually increases from the side ofthe incident side optical fiber 2620 toward the side of the graded indexslab waveguide 2610. The graph in FIG. 26 shows the refractive indexdistribution of the incident side optical fiber 2620 side end surface2641 of the incident side beam converter 2640 which refractive index isdesignated A and the refractive index distribution of the graded indexslab waveguide 2610 side end surface 2642 of the incident side beamconverter 2640 which refractive index distribution is designated B. Asis apparent from the graph, the refractive index distribution A changesmore gently than the refractive index distribution B.

The first exit side beam converter 2650 is disposed between the exitsurface 2613 of the graded index slab waveguide 2610 and the incidentside end surface of the first exit side optical fiber 2630. The secondexit side beam converter 2655 is disposed between the exit surface 2613of the graded index slab waveguide 2610 and the incident side endsurface of the second exit side optical fiber 2635. The first exit sidebeam converter 2650 and the second exit side beam converter 2655 aresubstantially cylindrical, and have a refractive index distribution suchthat the refractive index is the maximum at the central axis of thecylinder and decreases toward the periphery. The refractive indexdistributions of the first exit side beam converter 2650 and the secondexit side beam converter 2655 are such that the refractive index is themaximum at the center and changes toward the periphery substantiallyalong the quadratic function.

The first exit side beam converter 2650 has a refractive indexdistribution such that the change gradually increases from the side ofthe first exit side optical fiber 2630 toward the side of the gradedindex slab waveguide 2610. The second exit side beam converter 2655 hasa refractive index distribution such that the change gradually increasesfrom the side of the second exit side optical fiber 2631 toward the sideof the graded index slab waveguide 2610. The manner of the change is areversed one of the distribution of the above-described incident sidebeam converter 2640.

By the above structure, the multi-mode signal beam having exited fromthe incident side optical fiber 2620 is incident on the incident sidebeam converter 2640 and transmitted in the direction of the length. Theincident side beam converter 2640 converts the mode field (beam spotdiameter) of the incident side optical fiber 2620 into a small modefield (beam spot diameter) according to a change, in the direction ofthe length, of the refractive index distribution. The signal beam whosemode field has been converted into a small one is incident, as anincident beam, on the central position in the direction of the width onthe incident surface of the graded index slab waveguide 2610. Asdescribed in the first embodiment, the graded index slab waveguide 2610transmits the incident beam in the direction of the length, forms twoimages in the vicinity of the exit surface according to the self-imagingprinciple, and emits these images as exiting beams.

The emitted two signal beams are incident on the first exit side beamconverter 2650 and the second exit side beam converter 2655,respectively, and transmitted in the direction of the length. The firstexit side beam converter 2650 converts the mode field (beam spotdiameter) of the graded index slab waveguide 2610 into a large modefield (beam spot diameter) according to a change, in the direction ofthe length, of the refractive index distribution. The second exit sidebeam converter 2655 converts the mode field (beam spot diameter) of thegraded index slab waveguide 2610 into a large mode field (beam spotdiameter) according to a change, in the direction of the length, of therefractive index distribution. The signal beams whose mode fields havebeen converted into large ones are incident on the first exit sideoptical fiber 2630 and the second exit side optical fiber 2635,respectively, and then, transmitted.

As described above, since the optical device according to the fourteenthembodiment is provided with the beam converters that convert the modefields of the signal beams that are incident and exit on and from thegraded index slab waveguide, an incident beam having a small mode fieldcan be made incident on the graded index slab waveguide 2610.Consequently, according to the self-imaging principle, the mode field ofthe exiting beam can be made small.

Conventionally, when an optical fiber where the proportion of the corediameter (mode field, beam spot diameter) is large with respect to thewidth of the graded index slab waveguide like a POF is connected,according to the self-imaging principle, a beam with a large beam spotdiameter having the same profile as the incident beam is outputted onthe exit side, so that it is difficult to increase the distance betweenthe exiting beams. Consequently, the number of splits of the signal beamcannot be increased. On the other hand, in the optical device accordingto the fourteenth embodiment, since the mode field of the exiting beamcan be reduced, the number of splits can be easily increased.

When the mode field of the incidence is reduced, the distance between aplurality of exiting beams formed by the splitting is also reduced. Forthis, an optical fiber with a large core diameter can be connected tothe output side by angling the output side beam converter so that theoutput position can be parallelly moved or making it S-shaped by gentlycurving it. While in the above-described example, the graded index slabwaveguide 2610 is for use in the optical device that splits one beaminto two beams according to the first embodiment, it is to be noted thatthe straight sheet bus, the cross sheet bus, the star coupler, theoptical switch and the like described in the other embodiments areapplicable. In this case, the numbers of incident and exit side opticalfibers and incident and exit side beam converters are adjusted accordingto the number of incident and exiting signal beams.

FIG. 27 is a perspective view showing the general outline of thestructure of an optical device according to a first modification of thefourteenth embodiment of the present invention. Since the optical deviceaccording to the first modification of the fourth embodiment hassubstantially the same structure as the optical device of the fourteenthembodiment, only different parts will be described. Moreover, the samereference numerals indicate the same elements.

The optical device according to the first modification of the fourteenthembodiment is provided with a graded index slab waveguide 2710 where N=5in the optical device that splits one beam into a number, N, of beamsdescribed in the second embodiment. The optical device according to thefirst modification of the fourteenth embodiment is provided with anincident side beam converter 2740 comprising a graded index waveguidehaving a refractive index distribution such that the highest refractiveindex is provided at the center in the direction of the thickness andthe refractive index decreases substantially along a quadratic functiononly in the direction of the thickness. Moreover, the optical deviceaccording to the first modification of the fourteenth embodiment isprovided with an exit side beam converter 2750 comprising five gradedindex waveguides each having a refractive index distribution such thatthe highest refractive index is provided at the center in the directionof the thickness and the refractive index decreases substantially alonga quadratic function only in the direction of the thickness.

The graded index waveguide of the incident side beam converter 2720 hasa configuration such that the size in the direction of the widthdecreases from the side of the incident optical fiber 2620 toward theside of the graded index slab waveguide 2710. The graded indexwaveguides of the exit side beam converter 2730 are graded index slabwaveguides having a configuration such that the size in the direction ofthe width decreases from the side of the exit side optical fibers 2630to side of the graded index slab waveguide 2710. As described above,even when the graded index waveguides whose sizes in the direction ofthe width change are used as the beam converters on the incident andexit sides, the mode field can be converted.

FIG. 28A is a top view showing the general outline of the structure ofan optical device according to a second modification of the fourteenthembodiment of the present invention. FIG. 28B is a cross-sectional viewshowing an example of an exit side beam converter 2850 of the opticaldevice according to the second modification of the fourteenth embodimentof the present invention. FIG. 28C is a cross-sectional view showinganother example of the exit side beam converter 2850 of the opticaldevice according to the second modification of the fourteenth embodimentof the present invention. Since the optical device according to thesecond modification of the fourteenth embodiment has substantially thesame structure as the optical device of the fourteenth embodiment, onlydifferent parts will be described. Moreover, the same reference numeralsindicate the same elements.

The optical device according to the second modification of thefourteenth embodiment is provided with a graded index slab waveguide2810 where N=4 in the optical device that splits one beam into a number,N, of beams described in the second embodiment. The optical deviceaccording to the second modification of the fourteenth embodiment isprovided with the incident side beam converter 2740 described in thefirst modification. The optical device according to the secondmodification of the fourteenth embodiment is provided with a discreteexit side beam converter 2850 that covers all the output side opticalfibers 2830. The exit side beam converter 2850 is an opticaltransmission line having a refractive index distribution such that thehighest refractive index is provided at the center corresponding to thecenter, in the direction of the width, of the graded index slabwaveguide 2810 and the refractive index decreases toward the peripherywithin a plane vertical to the direction of the length. The exit sidebeam converter 2850 is either circular or rectangular in cross section.FIG. 28B shows the exit side beam converter 2850 which is circular incross section, and FIG. 28C shows the exit side beam converter 2850which is rectangular in cross section. In each cross-sectional view, theexit side optical fibers 2830 are all disposed within the cross section.As described above, even when a discrete optical transmission linehaving a refractive index distribution is used as the beam converter,the mode field can be converted.

It is unnecessary that the structure of the beam converter be the samebetween on the incident and exit sides like the second modification, butstructures may be appropriately combined. For example, it may beperformed to apply the structure described in the second modificationfor the incident side beam converter and apply the structure describedin the first modification for the exit side beam converter.

FIG. 29 is a perspective view showing the general outline of thestructure of an optical device according to a third modification of thefourteenth embodiment of the present invention. Since the optical deviceaccording to the second modification of the fourteenth embodiment hassubstantially the same structure as the optical device according to thefirst modification of the fourteenth embodiment, only different partswill be described. Moreover, the same reference numerals indicate thesame elements.

In the optical device according to the third modification of thefourteenth embodiment of the present invention, part of the clad of eachexit side optical fiber 2930 is cut away in the direction of the widthto thereby reduce the distance between the adjoining optical fibers. Asdescribed away, by cutting away part of the clad, the amount ofdeformation, in the direction of the width, of the exit side beamconverter 2750 can be reduced.

Embodiments Associated with Manufacturing Methods

Hereinafter, methods of manufacturing the sheet-form graded index slabwaveguides described in the embodiments will be described. Examples ofthe method of manufacturing the graded index slab waveguides include thefollowing two:

A first manufacturing method is a method in which the graded index slabwaveguides are manufactured by laminating ultra-thin films havingdifferent refractive indices according to the refractive index change inthe direction of the thickness. Concrete examples of the firstmanufacturing method include a method adopting an epoxy, an acrylic, apolycarbonate or a polyimide resin. Since the refractive index ischanged by adjusting the amount of addition of fluorine, heavy hydrogen,sulfur or the like to these resins, ultra-thin films having variousrefractive indices can be manufactured.

A second manufacturing method is a method in which the composition, inthe direction of the thickness, of the optical transmission line ischanged so as to be suited for the refractive index distribution in thedirection of the thickness. Concrete examples of the secondmanufacturing method include the methods shown below.

(1) A method in which ions are implanted into a sheet-form glassmaterial and the distribution of the implanted ions is controlled insidethe glass to thereby form a refractive index distribution.

(2) A method in which when a sheet-form polysilane is cured, the oxygenconcentration is controlled and a distribution is provided to the oxygenconcentration inside the polysilane to thereby form a refractive indexdistribution.

(3) A method in which when a sheet-form perfluorinated resin is cured,the distributions of high-refractive-index low molecules andlow-refractive-index monomers are controlled inside the resin to therebyprovide a refractive index distribution. The method (3) in which arefractive index distribution is formed inside the perfluorinated resinis applicable to other resins.

Hereinafter, the method (2) of the second manufacturing method in whichthe graded index slab waveguides are formed by use of polysilane will bedescribed in detail. Polysilane is cured by ultraviolet exposure or heattreatment. At this time, part of the polysilane structure is oxidizedinto a siloxane structure having a lower refractive index when cured.Therefore, the refractive index of the cured polysilane can becontrolled by changing the ratio between the part cured while remainingthe polysilane structure without being oxidized and the part oxidizedinto a siloxiane structure when cured. For example, when cured byultraviolet irradiation under an environment where oxygen is included inthe atmosphere, polysilane is cured with the oxygen concentrationdecreasing from the surface irradiated with ultraviolet rays toward thecenter, so that a refractive index distribution structure such that therefractive index decreases from the inside where the oxygenconcentration is low toward the surface where the oxygen concentrationis high can be formed. As described above, by equally irradiating asheet-form polysilane with ultraviolet rays from above and below, arefractive index distribution centrosymmetrical in the direction of thethickness can be obtained.

Hereinafter, the method of manufacturing of the graded index slabwaveguide of the optical device will be described. FIG. 30 is anexplanatory view showing an example of the method of manufacturing thegraded index slab waveguide. In FIG. 30, first, a transparent formingdie 3002 is prepared that has a concave portion 3001 having the samedepth as the slab thickness D of the graded index slab waveguide 3010described in the first embodiment and corresponding to the size of theplurality of graded index slab waveguides 3010 (first step). Thetransparent forming die 3002 is formed of a material that is transparentwith respect to ultraviolet rays. Then, a polysilane 3003 is poured intothe concave portion 3001 of the transparent forming die 3002 so assubstantially not to overflow out of the concave portion 3001 (secondstep). This step is shown in FIG. 30A.

Then, the polysilane 3003 accumulated in the concave portion 3001 isirradiated with ultraviolet rays 3004 from above and below in thedirection of the thickness and heated at the same time. This step isshown in FIG. 30B. Then, the polysilane 3003 is cured (third step). Thisstep is shown in FIG. 30B.

After the polysilane 3003 is cured, cutting into a desired shape of thegraded index slab waveguide 3010 is performed (fourth step). In thismanner, a plurality of graded index slab waveguides can be manufactured.The transparent forming die 3002 after the cutting can be used as asubstrate 3005 of the graded index slab waveguide 3010 as it is.Needless to say, the substrate 3005 may be removed.

The side walls of the concave portion 3002 which are cut lastly are notnecessarily vertical but may be tapered. Moreover, the sections of thegraded index slab waveguide 3000 may be optically polished. Moreover,when a sheet-form polysilane having a predetermined thickness can beformed, the sidewalls are not always necessary.

FIG. 31 is an explanatory view showing another example of the method ofmanufacturing the graded index slab waveguide. In FIG. 31, first, atransparent forming die 3102 is prepared that has a concave portion 3001having the same depth as the slab thickness D of the graded index slabwaveguide 3110 described in the first embodiment and corresponding tothe size of a single graded index slab waveguides 3110 (first step). Thetransparent forming die 3102 is formed of a material that is transparentwith respect to ultraviolet rays. Then, a polysilane 3103 is poured intothe concave portion 3101 of the transparent forming die 3102 so assubstantially not to overflow out of the concave portion 3101 (secondstep). This step is shown in FIG. 31A.

Then, the polysilane 3103 accumulated in the concave portion 3101 isirradiated with ultraviolet rays 3004 from above and below in thedirection of the thickness and is heated at the same time. This step isshown in FIG. 31B. Then, the polysilane 3103 is cured (third step). Thisstep is shown in FIG. 31B.

After the polysilane 3103 is cured, parts corresponding to the incidentand exit end surfaces are cut into the shape of the graded index slabwaveguide 3110 (fourth step). In this manner, the graded index slabwaveguide can be manufactured. The transparent forming die 3102 afterthe cutting can be used as a substrate 3105 of the graded index slabwaveguide 3110 as it is. Needless to say, the substrate 3005 may beremoved.

While the incident and exit end surfaces of the concave portion whichare cut and deleted lastly are not necessarily vertical but may betapered, it is desirable that the side surfaces 3106 in the direction ofthe width be vertical surfaces of not more than 10°. Moreover, while theincident and exit surfaces of the concave portion may be formed only inthe vicinity of the incident and exit positions as well as being cut anddeleted or the sections may be optically polished, when the thickness ofthe transparent forming die 3102 in the direction of the beam incidentand exit surfaces is not more than 10 μm, the transparent forming die3102 itself may be cut or polished into the incident and exit surfaces.

As described above, by accumulating the resin poured into the concaveportion provided in the transparent forming die, the film thickness canbe made arbitrarily thick even in the case of a resin with a lowviscosity. Therefore, an optical transmission line in which an opticalfiber with a large core diameter can be used on the incident and exitsides can be handled.

FIG. 32 is an explanatory view explaining the mechanism of therefractive index distribution using polysilane. As mentioned previously,polysilane is changed into a siloxane structure (FIG. 32(e)) having alower refractive index by the oxidization, at the time of curing, thatoccurs due to ultraviolet exposure or heat treatment. For this reason, arefractive index distribution can be provided by controlling the ratiobetween the polysilane structure (FIG. 32(d)) part that is not oxidizedand the siloxiane structure part that occurs due to oxidization. As isapparent from the figures, polysilane is disposed in an oxygenatmosphere (FIG. 32A) and either ultraviolet exposure or heating isperformed (FIG. 32B), whereby a mold is obtained in which the ratio ofthe polysilane structure is high in the central portion where the oxygenconcentration is low and the ratio of the siloxane structure is high inthe surface part where the oxygen concentration is high (FIG. 32C).

When the film thickness of the polysilane is not more than 50 μm, theoxygen concentration decreases from the surface toward the inside in thepolysilane due to the oxygen in the atmosphere. For this reason, arefractive index distribution is naturally formed such that therefractive index decreases from the inside where the oxygenconcentration is low toward the surface where the oxygen concentrationis high. Moreover, when the film thickness of the polysilane is not lessthan 50 μm, the refractive index distribution at the time of oxidizationcan be arbitrarily controlled by previously diffusing oxygen or an oxideinto the polysilane before cured in a predetermined distribution inaddition to the oxygen in the atmosphere.

Moreover, by oxidizing the polysilane symmetrically from both surfaces,a refractive index distribution symmetrical with respect to the centerin the direction of the thickness can be formed. However, whenultraviolet rays are applied from the substrate side in the case of thecuring by ultraviolet exposure, a material that is transparent withrespect to ultraviolet rays, for example, quartz or glass such as Pyrexis used, and when ultraviolet rays are applied from the substrate side,a material that is opaque with respect to ultraviolet rays such assilicon or resin may be used in addition to glass.

FIG. 33 is an explanatory view explaining a method of manufacturing theoptical device according to the first modification of the fourteenthembodiment of the present invention. Hereinafter, a method ofmanufacturing the optical device will be described with the firstmodification according to the fourteenth embodiment of the presentinvention as an example.

In a transparent forming die 3301, a concave portion 3302 correspondingto the graded index multi-mode waveguide, a concave portion 3303corresponding to the incident side beam converter and a concave portion3304 corresponding to the exit side beam converter are previouslyformed. Moreover, in the transparent forming die 3301, a V-groove 3305for positioning an incident side optical fiber 3310 and a V-groove 3306for positioning an exit side optical fiber 3311 are formed. A polysilane3320 is poured into the concave portions of the transparent forming die3301. After poured, the polysilane 3320 accumulated in the concaveportions is irradiated with ultraviolet rays from above and below andheated at the same time to thereby cure the polysilane 3301. Lastly,optical fibers are disposed in the V-groove 3305 and the V-groove 3306to manufacture the optical device.

However, it is desirable that the side walls 3330 that determine thedirection of the width of the concave portion 3302 be vertical surfacesof not more than 100. As described above, by accumulating the resinpoured into the concave portions provided in the transparent formingdie, the film thickness can be made arbitrarily thick even in the caseof a resin with a low viscosity. Therefore, even a case where an opticalfiber with a large core diameter is used for optical transmission lineson the incident and exit sides can be handled.

While the transparent forming die is used as part of the optical deviceas the substrate of the graded index slab waveguide in the aboveexample, the cured polysilane may be released from the forming die. Byreleasing the polysilane from the forming die, the transparent formingdie can be reused, so that the manufacturing cost of the transparentforming die can be reduced.

FIGS. 34 and 35 are explanatory views explaining another example of themethod of manufacturing the optical device according to the firstmodification of the fourteenth embodiment of the present invention. InFIG. 34, in a transparent forming die 3401, a concave portion 3402corresponding to the graded in dexmulti-mode waveguide, a concaveportion 3403 corresponding to the incident side beam converter and aconcave portion 3404 corresponding to the exit side beam converter arepreviously formed. With this transparent forming die 3401, a gradedindex slab waveguide 3410 in which incident and exit side beamconverters are integrally formed is formed by a previously-describedmethod such as ultraviolet exposure. The graded index slab waveguide3410 is released from the transparent forming die 3401 after curing.

Then, in FIG. 35, an assembly die 3501 is prepared in which a concaveportion 3502 corresponding to the graded index slab waveguide 3401, aV-groove 3503 for positioning an incident side optical fiber 3520 and aV-groove 3504 for positioning exit side optical fibers 3530 arepreviously formed. By disposing the graded index slab waveguide 3401,the incident side optical fiber 3520 and the exit side optical fiber3530 in the assembly die 3501, the optical device can be manufactured.

According to this manufacturing method, since the transparent formingdie used for the forming of the graded index slab waveguide 3401 can bereused by releasing the mold from the die, cost can be reduced.Moreover, since the positioning adjustment of the incident and exit sidebeam converters is unnecessary, the productivity at the time ofmanufacturing can be improved. Moreover, since the assembly die 3501does not require ultraviolet exposure in the manufacturing process, thelimitation on the material is small, so that a low-priced die materialcan be selected. Moreover, since the use of the assembly die 3501facilitates the position adjustment of the incident and exit sideoptical fibers, the productivity at the time of manufacturing can beimproved.

While in the exit side beam converters of FIGS. 34 and 35, the opticalaxes of the exit side optical fiber side and the GI multi-mode slabwaveguide side coincide with each other, the present invention is notlimited thereto. As described in the first modification of thefourteenth embodiment, by setting the distances between the exit sidebeam converters so as to gradually increase from the graded index slabwaveguide toward the exit side optical fibers, the configuration of thegraded index slab waveguide can be reduced.

It is to be noted that the above-described manufacturing method isapplicable not only to optical devices that split one beam into anumber, N, of beams but also to optical devices such as the straightsheet bus, the cross sheet bus, the optical switch and the star couplerdescribed in the embodiments.

Fifteenth Embodiment

FIG. 36A is a perspective view of a multi-mode interference 1×2 splitter5100 according to a fifteenth embodiment of the present invention. FIG.36B is a front view of the multi-mode interference 1×2 splitter 5100. InFIG. 36A, the coordinate system is defined as shown in the figure, thedownward direction of the figure is defined as they-direction, therightward direction of the figure is defined as the z-direction, and thedirection vertical to the y-direction and the z-direction is defined asthe x-direction.

The multi-mode interference 1×2 splitter 5100 is provided with asheet-form optical transmission line 5101, an incident portion 5104, anexit portion 5105, an exit portion 5106, an electric purpose substrate5107 and an electric purpose substrate 5108. The sheet-form opticaltransmission line 5101 has a three-layer structure in which the electricpurpose substrate 5107, the sheet-form optical transmission line 5101and the electric purpose substrate 5108 are laminated in this order inthe positive direction of the y-direction.

The sheet-form optical transmission line 5101 is a two-dimensionaloptical transmission line whose thickness in the y-direction (thedirection of the thickness) is d and that is parallel to the z-x plane.The sheet-form optical transmission line 5101 traps the externallyincident signal beam in the y-direction and can transmit it in thez-direction (transmission direction). The sheet-form opticaltransmission line 5101 has a reflecting surface 5102 and a reflectingsurface 5103 at both ends in the z-direction.

The reflecting surface 5102 is formed at one end in the z-direction. Thereflecting surface 5102 is a reflecting surface inclined 45° withrespect to the z-x plane so that the signal beam incident in thepositive direction of the y-direction is bent in the positive directionof the z-direction.

The reflecting surface 5103 is formed at the other end in thez-direction which end is opposite to the incident side. The reflectingsurface 5103 is a reflecting surface inclined 45° with respect to thez-x plane so that the signal beam transmitted in the positive directionof the z-direction is bent in the negative direction of the y-direction.

The sheet-form optical transmission line 5101 has a refractive indexdistribution in the y-direction. In the sheet-form optical transmissionline 5101, the highest refractive index n_(max) is provided on a surface(hereinafter, referred to as a central portion 5101 a) parallel to thezx direction and situated in a position of d/2 which is half thethickness in the y-direction. The sheet-form optical transmission line5101 has a refractive index distribution such that with the centralportion 5101 a as the symmetry plane, the refractive index continuouslydecreases from the central portion 5101 a toward the electric purposesubstrate 5107 and the electric purpose substrate 5108.

Moreover, in the sheet-form optical transmission line 5101, therefractive index within a plane parallel to the z-x plane is always thesame. That is, the sheet-form optical transmission line 5101 has arefractive index distribution only in the y-direction and has norefractive index distribution in the other directions.

The electric purpose substrate 5107 and the electric purpose substrate5108 are flat. The electric purpose substrate 5107 includes a lightemitting element 5110, a light receiving element 5111 and a lightreceiving element 5112.

The light emitting element 5110 is a vertical cavity surface emittinglaser for generating a signal beam. The vertical cavity surface emittinglaser is disposed so that the laser serving as the signal beam isoscillated in the positive direction of the y-direction.

Moreover, the light receiving element 5111 and the light receivingelement 5112 are photodiodes that receive the signal beam. Thephotodiodes are disposed so as to receive the signal beam transmitted inthe negative direction of the y-direction. On the electric purposesubstrate 5107 and the electric purpose substrate 5108, non-illustratedother electric parts and optical parts necessary for driving the opticaldevice are mounted.

The electric purpose substrate 5107 has a throughhole, which is acylindrical through hole, in a position corresponding to the lightemitting element 5110. Inside the through hole, the cylindrical incidentportion 5104 is formed. Moreover, the electric purpose substrate 5107has a through hole, which is a cylindrical through hole, in a positioncorresponding to the light receiving element 5111.

Inside the through hole, the cylindrical exit portion 5105 is formed.Likewise, the electric purpose substrate 5107 has a through hole, whichis a cylindrical through hole, in a position corresponding to the lightreceiving element 5112. Inside the through hole, the cylindrical exitportion 5106 is formed.

The incident portion 5104 is made of the same material as the materialof the sheet-form optical transmission line 5101. The incident portion5104 has a refractive index distribution axisymmetrical with respect tothe central axis of the cylinder, and has a refractive indexdistribution such that the highest refractive index n_(max) is providedat the central axis of the cylinder and the refractive index does notcontinuously increase with distance from the central axis toward theperiphery symmetrically with respect to the central axis. The length, inthe y-direction, of the incident portion 5104 is determined so that thesignal beam is incident on the sheet-form optical transmission line 5101as a parallel beam.

The incident portion 5104, the exit portion 5105 and the exit portion5106 have the same structure. Moreover, the incident portion 5104, theexit portion 5105 and the exit portion 5106 are all bonded to thesheet-form optical transmission line 5101. The incident portion 5104,the exit portion 5105 and the exit portion 5106 are formed inpredetermined positions according to a condition of the self-imagingprinciple of the multi-mode interference. The condition of theself-imaging principle of the multi-mode interference will be describedlater.

In the above-described structure, the signal beam oscillated from thelight emitting point of the vertical cavity surface emitting laser ofthe light emitting element 5110 is incident on the incident portion 5104and travels in the positive direction of the y-direction. Then, thesignal beam is incident on the sheet-form optical transmission line 5101from the incident portion 5104, is bent in the positive direction of thez-direction by the reflecting surface 5102, and propagates in thesheet-form transmission line 5101. The signal beam is diffused in thex-direction and transmitted in multiple modes in the positive directionof the z-direction in the sheet-form optical transmission line 5101.Then, the signal beam is bent in the negative direction of they-direction by the reflecting surface 5103.

Since the incident portion 5104, the exit portion 5105 and the exitportion 5106 are formed in the predetermined positions according to thecondition of the self-imaging principle of the multi-mode interferencedescribed later, the signal beam is split into two beams of equal energyin the positions of the exit portion 5105 and the exit portion 5106, andthe two beams are incident on the exit portion 5105 and the exit portion5106, respectively.

The signal beam incident on the exit portion 5105 exits from the exitportion 5101 and is detected at the light receiving surface of thephotodiode of the light receiving element 5111. Likewise, the signalbeam incident on the exit portion 5106 exits from the exit portion 5106and is detected at the light receiving portion of the photodiode of thelight receiving element 5112.

As described above, the exit portion 5106 has the same structure as theexit portion 5105, and is disposed in the predetermined positionaccording to the condition of the self-imaging principle of themulti-mode interference. For this reason, the exit portion 5106 isequivalent to the exit portion 5105, and has the same optical property.Therefore, the description given below is based on only the exit portion5105, and the description of the exit portion 5106 is omitted because itis the same as that of the exit portion 5105.

The optical axes, of the signal beam transmitted inside, of the incidentportion 5104, the exit portion 5105 and the exit portion 5106 are allparallel to the y-direction, and orthogonal to the z-direction which isthe signal beam transmission direction of the sheet-form opticaltransmission line 5101. Therefore, the incident portion 5104, the exitportion 5105 and the exit portion 5106 are all nonparallel incidentportions.

FIG. 37 is a cross-sectional view of a part, where the signal beam istransmitted, of the multi-mode interference 1×2 splitter 5100 accordingto the fifteenth embodiment of the present invention. FIG. 2 is across-sectional view in which the D-H side of the cross section, takenon a plane including the C-D-G-H plane in FIG. 36A, of the sheet-formoptical transmission line 5101 and the incident portion 5104 and the E-Iside of the cross section, taken on a plane including the E-F-I-J planein FIG. 36A, of the sheet-form optical transmission line 5101 and theexit portion 5105 are connected together.

Here, the plane including the C-D-G-H plane is a plane parallel to they-z plane and including the central axis of the incident portion 5104.Moreover, the plane including the E-F-I-J is a plane parallel to the y-zplane and including the central axis of the exit portion 5106.

In FIG. 37, the same elements are denoted by the same reference numeralsas those of FIG. 36. In FIG. 37, the light emitting point of the lightemitting element 5110 is a light emitting point 5110 a, and the lightreceiving point on the light receiving surface of the light receivingelement 5111 is a light receiving point 5111 a.

In the multi-mode interference 1×2 splitter 5100 according to thefifteenth embodiment, the signal beam is transmitted only in thepositive direction of the y-direction in the sheet-form opticaltransmission line 5101. For this reason, when the phase condition in thesignal beam transmission direction is discussed, it is unnecessary toconsider the signal beam diffusion in the x-direction. This is becausethe signal beam diffusion in the x-direction which is caused by a changein the signal beam intensity distribution due to the multi-modeinterference involves no energy propagation, and is always in phase inthe x-direction of the signal beam.

Therefore, in FIG. 37, when the phase condition in the transmissiondirection is discussed, the D-E-H-I plane parallel to the x-y plane inFIG. 36 may be ignored, and the optical path described in a medium inwhich the D-H side of the C-D-G-H plane and the E-I side of the E-F-I-Jplane of FIG. 37 are connected together is equivalent to the opticalpath of the signal beam. As described above, it is assumed that when theterm optical path or optical path length is used, the diffusion in thex-direction is ignored in the embodiments described below.

In FIG. 37, the signal beam oscillated from the light emitting point5110 a which signal beam is a divergent beam includes light beams thattravel along various optical paths. Of the signal beam oscillated fromthe light emitting point 5110 a, particularly, two optical paths A and Bincident on the farthest positions from the optical axis of the signalbeam will be examined. The optical path A is symmetrical to the opticalpath B with respect to the optical axis of the signal beam incident onthe incident portion 5104. In FIG. 37, the optical path A is shown bysolid lines, and the optical path B is shown by dotted lines.

In FIG. 37, the signal beam oscillated from the light emitting point5110 a in the positive direction of the y-direction is incident on theincident portion 5104 as a divergent beam. The incident portion 5104 hasa refractive index distribution such that the refractive index does notcontinuously increase with distance from the central axis to theperiphery symmetrically with respect to the central axis as mentionedabove. For this reason, of the signal beam incident on the incidentportion 5104, light beams incident on the incident portion 5104 atangles other than 90° are not linearly transmitted but travels whilemeandering.

That is, the light beam transmitted along the optical path A istransmitted from a region where the refractive index is high to a regionwhere the refractive index is low, and is gradually bent parallelly tothe y-direction. The light beam transmitted along the optical path B isalso transmitted from a region where the refractive index is high to aregion where the refractive index is low, and is gradually bentparallelly to the y-direction.

The length, in the y-direction, of the incident portion 5104 isdetermined so that the signal beam becomes a parallel beam (collimatedbeam) when incident on the sheet-form optical transmission line 5101.That is, the physical length, in the y-direction, of the incidentportion 5104 is determined so that the optical path A is parallel to theoptical path B. Consequently, the signal beam is incident on thesheet-form optical transmission line 5101 as a parallel beam.

The optical path A vertically traverses the sheet-form opticaltransmission line 5101 to reach the reflecting surface 5102 and is bentin the positive direction of the z-direction by the reflecting surface5102. On the other hand, the optical path B immediately reaches thereflecting surface 5102 and is bent in the positive direction of thez-direction by the reflecting surface 5102. By the optical path A beingbent by the reflecting surface 5102, the signal beam is all transmittedin the positive direction of the z-direction of the sheet-form opticaltransmission line 5101. Then, the optical path A and the optical path Btravel while meandering according to the refractive index distribution.

The optical path A reaches the reflecting surface 5103, and is bent inthe negative direction of the y-direction by the reflecting surface5103. On the other hand, the optical path B parallelly incident in thepositive direction of the y-direction reaches the reflecting surface5103 later than the optical path A, and is bent in the negativedirection of the y-direction by the reflecting surface 5103.

At this time, the structure, in the z-direction, of the sheet-formoptical transmission line 5101 is determined so that the signal beambecomes a parallel beam when exiting from the exit portion 5105. Thatis, the physical length of the sheet-form optical transmission line 5105is determined so that the optical path A is parallel to the optical pathB. Consequently, the signal beam is incident on the exit portion 5105 asa parallel beam. Here, the optical axis of the signal beam transmittedthrough the exit portion 5105 is parallel to the y-direction, and isorthogonal to the z-direction which is the signal beam transmissiondirection of the sheet-form optical transmission line 5101. Thestructure of the sheet-form optical transmission line 5101 will bedescribed later in detail.

The exit portion 5105 has a refractive index distribution such that therefractive index does not continuously increase with distance from thecentral axis to the periphery symmetrically with respect to the centralaxis as mentioned previously. For this reason, of the signal beamincident on the exit portion 5105, signal beams incident on the partaway from the central axis are not linearly transmitted but travel whilemeandering.

The optical path A is transmitted from a region where the refractiveindex is low to a region where the refractive index is high, and isgradually bent in a direction that approaches the optical axis ofsymmetry. The optical path B is also transmitted from a region where therefractive index is low to a region where the refractive index is high,and is gradually bent in a direction that approaches the optical axis ofsymmetry.

The refractive index distribution and the physical length, in they-direction, of the exit portion 5101 are the same as those of theincident portion 5104. For this reason, the signal beam exits from theexit portion 5105 as a convergent beam, and is imaged at the lightreceiving point 5111 a.

The physical optical path length, on the optical path A, from theposition corresponding to the position where the optical path B reachesthe reflecting surface 5102 to the position where the optical path Areaches the reflecting surface 5102 is defined as LA1. The physicaloptical path length, on the optical path B, from the position where theoptical path B reaches the reflecting surface 5102 to the positioncorresponding to the position where the optical path A reaches thereflecting surface 5102 is defined as LB1.

Moreover, the physical optical path length, on the optical path A, fromthe position where the optical path A reaches the reflecting surface5103 to the position corresponding to the position where the opticalpath B reaches the reflecting surface 5103 is defined as L2A. Thephysical optical path length, on the optical path B, from the positioncorresponding to the position where the optical path A reaches thereflecting surface 5103 to the position where the optical path B reachesthe reflecting surface 5103 is defined as L2B.

Moreover, the physical distance from the position where the optical pathA reaches the reflecting surface 5102 to the position where the opticalpath A reaches the reflecting surface 5103 is defined as a transmissionlength L. The transmission length L corresponds to the physical lengthof the region where the signal beam is transmitted in the positivedirection of the z-direction.

Since the reflecting surface 5102 and the reflecting surface 5103 bothfunction as mirrors that bend the optical path 90 degrees, theirgeometries in the y- and z-directions are common. Therefore, thephysical optical path length L1A is equal to the physical optical pathlength L1B. Likewise, the physical optical path length L2A is equal tothe physical optical path length L2B.

However, the optical path length corresponding to the physical opticalpath length L1A does not coincide with the optical path lengthcorresponding to the physical optical path length L1B. This is because,since the optical path length is different after the reflection at thereflecting surface 5102, the phase of the light beam traveling along theoptical path A does not coincide with the phase of the light beamtraveling along the optical path B. That is, a phase difference occursbetween the optical path A and the optical path B. As described above,when a reflecting surface that is not vertical to the transmissiondirection is provided in a sheet-form optical transmission lineincluding a refractive index distribution, the reflecting surfacefunctions as an optical path length difference generating portion.

Since there is a phase difference between the optical path A and theoptical path B, the intensity peak position of the signal beamtransmitted inside the sheet-form optical transmission line 5101 isshifted. The refractive index that affects the signal beam while theoptical path A is transmitted along the optical path corresponding toL1A is higher than the refractive index that affects the signal beamwhile the optical path B is transmitted along the optical pathcorresponding to L1B.

Consequently, the optical path A is delayed in phase from the opticalpath B. Therefore, as is apparent from FIG. 37, the position where theseoptical paths intersect each other does not coincide with the centralportion 5101 a of the sheet-form optical transmission line 5101 but isshifted toward the positive side in the y-direction.

Moreover, the phase of the light beam transmitted in the z-direction inthe sheet-form optical transmission line 5101 is not disturbed by therefractive index distribution. Therefore, the phase difference betweenthe optical path A and the optical path B caused by the reflectingsurface 5102 is transmitted to the reflecting surface 5103 as it is. Atthe reflecting surface 5103, a phase difference is caused between theoptical path A and the optical path B by the same mechanism.

The phase difference caused by the reflecting surface 5102 isquantified. FIG. 38A is a cross section, taken on the plane includingthe C-D-G-H plane in FIG. 36A, of the sheet-form optical transmissionline 5101 and the incident portion 5104. FIG. 38B is a graph showing therefractive index distribution of the sheet-form optical transmissionline 5101. In FIG. 38B, the vertical axis coincides with they-direction, and the origin of the y-coordinate is the central portion5101 a.

Letting the refractive index distribution constant be g and therefractive index at the central portion 5101 a be n_(max), therefractive index distribution in the y-direction is defined by thequadratic function shown by the following (Expression 2):$\begin{matrix}{{n(y)} = {n_{\max}\left( {1 - \frac{g^{2}y^{2}}{2}} \right)}} & \left( {{Expression}\quad 9} \right)\end{matrix}$

In FIG. 38B, the horizontal axis represents the refractive index n(y)and the vertical axis represents the position coordinate, in they-direction, of the sheet-form optical transmission line 5101. Theorigin of the position is the central portion 5101 a of the sheet-formoptical transmission line 5101. As is apparent from FIG. 38B,(Expression 9) is a quadratic function that is convex upward, and therefractive index distribution is such that the refractive index at thecentral portion 5101 a is the highest refractive index n_(max) and therefractive index continuously and centrosymmetrically decreases withdistance from the central portion 5101 a in both the positive andnegative directions of the y-direction.

To convert the physical optical path length into an optical path length,the function of the refractive index distribution is integrated withrespect to the position. For the sake of simplification, it is assumedthat the position where the signal beam transmitted along the opticalpath A and the optical path B is reflected by the reflecting surface5102 is the position of the largest diameter of the sheet-form opticaltransmission line 5101.

In actuality, in the signal beam transmitted in the sheet-form opticaltransmission line 5101, a plurality of modes is excited in the directionof the width, and the effective refractive index differs among themodes. In the MMI, since the size in the direction of the length is afunction of the effective refractive index of the 0th-order mode beamexcited in the direction of the width, it is more convenient to replacethe highest refractive index n_(max) with the effective refractive indexno of the 0th-order mode beam excited in the direction of the width.Therefore, in the following discussion, the effective refractive indexno of the 0th-order mode beam excited in the direction of the width isused as the refractive index. The effective refractive index no isdetermined by the highest refractive index n_(max), the wavelength ofthe signal beam and the configuration of the sheet-form opticaltransmission line.

The optical path length corresponding to the physical optical pathlength L1A of the optical path A is equal to (Expression 9), being afunction of the refractive index, integrated with respect to theposition coordinate y from −d/2 to d/2. That is, the optical path lengthcorresponding to the physical optical path length L1A of the opticalpath A is equal to the area of the region a defined by the y-axis andthe graph of (Expression 9) representing the refractive indexdistribution in FIG. 38B.

Likewise, the optical path length corresponding to the physical opticalpath length L1B of the optical path B is equal to the value n(d/2), ofthe refractive index at a position d/2 in FIG. 38B, integrated withrespect to the position coordinate y from −d/2 to d/2. That is, theoptical path length corresponding to the physical optical path lengthL1B of the optical path B is equal to the area of the rectangular regionβ in FIG. 38B.

Therefore, the difference ΔL in optical path length between the opticalpath A and the optical path B caused by the reflecting surface 5102 isexpressed by the following (Expression 10). Moreover, the phasedifference A caused between the optical path A and the optical path B atthis time is expressed by the following (Expression 11). Here, theeffective refractive index no is used as the refractive index.$\begin{matrix}{{\Delta\quad L} = {{\int_{- \frac{d}{2}}^{\frac{d}{2}}{\left\{ {{n(y)} - {n\left( \frac{d}{2} \right)}} \right\}\quad{\mathbb{d}y}}} = {{2{\int_{0}^{\frac{d}{2}}{\left\{ {{n(y)} - {n\left( \frac{d}{2} \right)}} \right\}\quad{\mathbb{d}y}}}} = \frac{n_{0}g^{2}d^{3}}{12}}}} & \left( {{Expression}\quad 10} \right) \\{\Delta = {{\frac{2\pi}{\lambda}\Delta\quad L} = {\frac{n_{0}g^{2}d^{3}}{6\lambda}\pi}}} & \left( {{Expression}\quad 11} \right)\end{matrix}$

Results of concrete numerical calculations of the above-mentioned phasedifference Δ are shown in Table 1. In the calculations, the refractiveindex distribution coefficient g is set as a value that decreasesapproximately 1% from the center at d/2. Moreover, the effectiverefractive index corresponding to the refractive index at the centralportion 5101 a is set to n₀=1.5. TABLE 1 Diameter, in the direction ofthe refractive index distribution, of the sheet-form opticaltransmission line[ μm] 50 100 200 1000 Refractive index 5.6 2.8 1.4140.28 distribution coefficient g [mm⁻¹] Phase 1.18 π 2.35 π 4.71 π 23.5 πdifference Δ [radian]

As is apparent from Table 1, the phase difference between the opticalpath A and the optical path B caused at the reflecting surface 5102 isnot less than π radian. The signal beam cannot be made to exit with itsintensity distribution being unchanged unless the phase differencebetween the optical path A and the optical path B is zero.

Therefore, a method of compensating for the phase difference will bedescribed. First, a condition that is necessarily satisfied by thephysical length (hereinafter, referred to as transmission length) L fromthe position where the signal beam transmitted along the optical path Areaches the reflecting surface 5102 to the position where the signalbeam transmitted along the optical path A reaches the reflecting surface5103.

The signal beam incident on the sheet-form optical transmission line5101 from the incident portion 5104 and the signal beam exiting from thesheet-form optical transmission line 5101 to the exit portion 5105 areboth parallel beams. Moreover, when the refractive index distributioncoefficient g is provided, the light beam transmitted in the sheet-formoptical transmission line 5101 meanders with a period of 2π/g accordingto the refractive index distribution.

Therefore, to cause the signal beam incident as a parallel beam to exitas a parallel beam at the sheet-form optical transmission line 5101, thetransmission length L is set to an integral multiple of the period 2/g.That is, the transmission length L necessarily satisfies the following(Expression 12): $\begin{matrix}{L = {\frac{2\pi}{g}j\quad\left( {{j = 0},1,2,3,\ldots} \right)}} & \left( {{Expression}\quad 12} \right)\end{matrix}$

On the other hand, because of the refractive index distribution, theoptical path length corresponding to the physical transmission length Lof the optical path A is equal to the optical path length correspondingto the physical transmission length of the optical path B. Thedifference in overall optical path length between the optical path A andthe optical path B of the sheet-form optical transmission line 5101 canbe considered to be caused only at the reflecting surface 5102 and thereflecting surface 5103. As described above, the reflecting surface 5102and the reflecting surface 5103 are optical path length differencegenerating portions.

The difference ΔL_(total) in overall optical path length of thesheet-form optical transmission line 5101 is equal to the value of thefollowing (Expression 13) which is double the (Expression 10) calculatedwith respect to the reflecting surface 5102: $\begin{matrix}{{\Delta\quad L_{total}} = {{2\Delta\quad L} = \frac{n_{0}g^{2}d^{3}}{6}}} & \left( {{Expression}\quad 13} \right)\end{matrix}$

The above (Expression 13) means that the optical path length of theoptical path A is larger by the value of ΔL_(total) than the opticalpath length of the optical path B. Therefore, by making the value of(Expression 13) to coincide with an integral multiple of the wavelengthof the signal beam, the difference between the phase of the optical pathA and the phase of the optical path B can be made zero. That is, thecondition that makes zero the phase difference between the optical pathA and the optical path B is a condition expressed by the following(Expression 14): $\begin{matrix}{{\Delta\quad L_{total}} = {\frac{n_{0}g^{2}d^{3}}{6} = {k\quad\lambda\quad\left( {{k = 1},2,3,\ldots} \right)}}} & \left( {{Expression}\quad 14} \right)\end{matrix}$

As described above, the phase difference between the optical path A andthe optical path B is a natural multiple of the wavelength k of thesignal beam by structuring the sheet-form optical transmission line 5101so that the difference in optical path length between the optical path Aand the optical path B caused in the entire sheet-form opticaltransmission line 5101 satisfies (Expression 14). Consequently, thephase difference between the optical path A and the optical path B doesnot occur.

The sheet-form optical transmission line 5101 is designed as follows:First, the transmission length L is determined by (Expression 12). Bythis, the refractive index distribution coefficient g is determined.Then, (Expression 14) is adjusted by use of the determined refractiveindex distribution coefficient g and the preprovided signal beamwavelength λ.

The parameters for the adjustment are the refractive index n_(max) atthe central portion 5101 a and the thickness d in the y-direction. Theeffective refractive index no of the 0th-order mode beam excited in thedirection of the width can be changed by changing the refractive indexn_(max) at the central portion 5101 a and the thickness d in they-direction. When the adjustment cannot be made, the refractive indexdistribution coefficient g is changed and the transmission length L isagain determined by (Expression 12). By repeating this optimizationdesign, a desired sheet-form optical transmission line 5101 can beobtained.

As described above, in the optical device according to the fifteenthembodiment, the difference in optical path length between the opticalpath A and the optical path B is a natural multiple of the signal beamwavelength λ. Therefore, the phase difference between the optical path Aand the optical path B is the same between before the incidence on theoptical transmission line and the exit from the optical transmissionline. Consequently, in the optical device according to the fifteenthembodiment, the waveform at the time of the incidence on the opticaltransmission line and the waveform at the time of the exit therefrom canbe made to coincide with each other, so that the signal beam can be madeto exit from the optical transmission line without any loss.

Moreover, in the optical device according to the fifteenth embodiment,since the optical axis of the signal beam incident on the opticaltransmission line and the optical axis of the signal beam exiting fromthe optical transmission line are both orthogonal to the z-direction,the outside and the optical transmission line can be easily coupledtogether. In particular, when optical parts such as a light emittingelement that emits the signal beam that is incident on the opticaltransmission line and a light receiving element that receives the signalbeam having exited from the optical transmission line are coupled to theoptical transmission line, the optical parts can be easily mounted.

Moreover, in the optical device according to the fifteenth embodiment,the optical path A and the optical path B include two optical pathlength difference generating portions where the optical path lengthdifference is caused, and the sum of the optical path length differencescaused by the two optical path length difference generating portions isequal to a natural multiple of the signal beam wavelength. By thisstructure, the phase difference between the two optical paths can bemade zero.

Moreover, the optical device according to the fifteenth embodimentincludes a sheet-form optical transmission line capable of trapping thesignal beam in the y-direction, and the sheet-form optical transmissionline has a refractive index distribution such that the refractive indexat the central portion where the thickness in the y-direction is half isthe highest and the refractive index does not increase with distancefrom the center in a first direction. By this structure, the modedispersion is suppressed by the refractive index distribution, and thesignal beam can be transmitted.

Moreover, in the optical device according to the fifteenth embodiment,the sheet-form optical transmission line includes the reflecting surface5102 for bending, in the z-direction, the optical axis of the signalbeam incident from a direction not parallel to the z-direction and thereflecting surface 5103 for bending, in the direction not parallel tothe z-direction, the optical axis of the signal beam transmitted in thez-direction. In this case, the reflecting surface 5102 and thereflecting surface 5103 are optical path length difference generatingportions.

By this structure, the signal beam incident on the optical transmissionline from the direction not parallel to the z-direction can be easilymade incident on the optical transmission line. Moreover, the signalbeam exiting from the optical transmission line in the direction notparallel to the z-direction can be easily made to exit from the opticaltransmission line.

Moreover, in the optical device according to the fifteenth embodiment,in the sheet-form optical transmission line, the physical optical pathlength from the position where the signal beam is all bent in thez-direction by the reflecting surface 5102 to the position immediatelybefore the signal beam is all incident on the reflecting surface 5103 isequal to j times (j=0,1,2,3, . . . ) the period of meandering of theoptical path along which the signal beam is transmitted while meanderingbased on the refractive index distribution. By this structure, theintensity distribution of the signal beam is the same between on theincident side and on the exit side.

Sixteenth Embodiment

Next, a sixteenth embodiment of the present invention will be described.In the sixteenth embodiment, descriptions of the same parts as those ofthe fifteenth embodiment are omitted and only different parts will bedescribed. A multi-mode interference 1×2 splitter 5200 of the sixteenthembodiment has approximately the same structure as the multi-modeinterference 1×2 splitter 5100 shown in FIG. 36, and is different onlyin the structure of a sheet-form optical transmission line 5201.

FIG. 39 is a cross-sectional view of a part, where the signal beam istransmitted, of the multi-mode interference 1×2 splitter 5200 accordingto the sixteenth embodiment of the present invention. FIG. 39 is across-sectional view of the multi-mode interference 1×2 splitter 5200taken on the same place as that in the case of the multi-modeinterference 1×2 splitter 5100 according to the fifteenth embodimentshown in FIG. 36 and FIG. 37. In FIG. 39, the incident portion 5104, theexit portion 5105, the reflecting surface 5102 and the reflectingsurface 5103 all have the same structures as those of the multi-modeinterference 1×2 splitter 5100 according to the fifteenth embodiment.

The sheet-form optical transmission line 5201 has a refractive indexdistribution in the y-direction. The sheet-form optical transmissionline 5201 has the highest refractive index n_(max) at the centralportion 5101 a. The sheet-form optical transmission line 5201 has arefractive index distribution that satisfies (Expression 9) with thecentral portion 5101 a as the symmetry plane. Moreover, the sheet-formoptical transmission line 5201 has a refractive index distribution onlyin the y-direction, and has no refractive index distribution in theother directions. Moreover, the transmission length L of the sheet-formoptical transmission line 5201 satisfies the following (Expression 15):$\begin{matrix}{L = {\frac{2\pi}{g}\left( {j + 0.5} \right)\quad\left( {{j = 0},1,2,\ldots} \right)}} & \left( {{Expression}\quad 15} \right)\end{matrix}$

(Expression 15) means that the transmission length L is (an integer+0.5)times the period of meandering when the beam is transmitted through thesheet-form optical transmission line 5201. When the transmission lengthL satisfies (Expression 15), the period of meandering of the opticalpath A and the optical path B is shifted by half the period compared tothat at the time of incidence.

The optical path A is reflected at the side, the farthest from theincident portion 5104, of the reflecting surface 5102 to be bent in thepositive direction of the z-direction and transmitted, and then,reflected at the side, the closest to the exit portion 5105, of thereflecting surface 5103. Likewise, the optical path B is reflected atthe side, the closest to the incident portion 5104, of the reflectingsurface 5102 to be bent in the positive direction of the z-direction andtransmitted, and then, reflected at the side, the farthest from the exitportion 5105, of the reflecting surface 5103.

Here, the physical optical path lengths L1A, L2A, L1B and L2B aredefined equally to the case of the fifteenth embodiment. Moreover, thetransmission length L is also defined equally to the case of thefifteenth embodiment. In the case of the sixteenth embodiment, theoptical path length corresponding to the physical optical path lengthL1A of the optical path A is equal to the optical path lengthcorresponding to the physical optical path length L2B of the opticalpath B. Moreover, the optical path length corresponding to the physicaloptical path length L2A of the optical path A is equal to the opticalpath length corresponding to the physical optical path length L1B of theoptical path B.

On the other hand, because of the refractive index distribution, theoptical path length corresponding to the physical transmission length Lof the optical path A is equal to the optical path length correspondingto the physical transmission length of the optical path B. Therefore,the difference in overall optical path length between the optical path Aand the optical path B of the sheet-form optical transmission line 5201is zero. Similarly to the case of the fifteenth embodiment, when theoptical path length is ΔL_(total), the following (Expression 16) holds:ΔL _(total)=0  (Expression 16)

That is, the difference between the optical path length of the opticalpath A and the optical path length of the optical path B is zero. Sincethe difference in optical path length is zero, no phase differenceoccurs between the optical path A and the optical path B. As describedabove, the phase difference between the optical path A and the opticalpath B is zero when the transmission length L of the sheet-form opticaltransmission line 5201 is set so as to satisfy (Expression 15).

As described above, in the optical device according to the sixteenthembodiment, the difference in optical path length between the opticalpath A and the optical path B is zero. Therefore, the phase differencebetween the optical path A and the optical path B is the same betweenbefore the incidence on the optical transmission line and after the exitfrom the optical transmission line. Consequently, in the optical deviceaccording to the sixteenth embodiment, the waveform at the time of theincidence on the optical transmission line and the waveform at the timeof the exit therefrom can be made to coincide with each other, so thatthe signal beam can be made to exit from the optical transmission linewithout any loss.

Moreover, in the optical device according to the sixteenth embodiment,since the optical axis of the signal beam incident on the opticaltransmission line and the optical axis of the signal beam exiting fromthe optical transmission line are both orthogonal to the z-direction,the outside and the optical transmission line can be easily coupledtogether. In particular, when optical parts such as a light emittingelement that emits the signal beam that is incident on the opticaltransmission line and a light receiving element that receives the signalbeam having exited from the optical transmission line are coupled to theoptical transmission line, the optical parts can be easily mounted.

Moreover, in the optical device according to the sixteenth embodiment,the optical path A and the optical path B include two optical pathlength difference generating portions where the optical path lengthdifference is caused, and the sum of the optical path length differencescaused by the two optical path length difference generating portions iszero. By this structure, the phase difference between the two opticalpaths can be made zero.

Moreover, the optical device according to the sixteenth embodimentincludes a sheet-form optical transmission line capable of trapping thesignal beam in the y-direction, and the sheet-form optical transmissionline has a refractive index distribution such that the refractive indexat the central portion where the thickness in the y-direction is half isthe highest and the refractive index does not increase with distancefrom the center in a first direction. By this structure, the modedispersion is suppressed by the refractive index distribution, and thesignal beam can be transmitted.

Moreover, in the optical device according to the sixteenth embodiment,the sheet-form optical transmission line includes the reflecting surface5102 for bending, in the z-direction, the optical axis of the signalbeam incident from a direction not parallel to the z-direction and thereflecting surface 5103 for bending, in the direction not parallel tothe z-direction, the optical axis of the signal beam transmitted in thez-direction. Further, the reflecting surface 5102 and the reflectingsurface 5103 are optical path length difference generating portions.

By this structure, the signal beam incident on the optical transmissionline from the direction not parallel to the z-direction can be easilymade incident on the optical transmission line. Moreover, the signalbeam exiting from the optical transmission line in the direction notparallel to the z-direction can be easily made to exit from the opticaltransmission line.

Moreover, in the optical device according to the sixteenth embodiment,in the sheet-form optical transmission line, the physical optical pathlength from the position where the signal beam is all bent in thez-direction by the reflecting surface 5102 to the position immediatelybefore the signal beam is all incident on the reflecting surface 5103 isequal to (j+0.5) times (j=0,1,2,3, . . . ) the period of meandering ofthe optical path along which the signal beam is transmitted whilemeandering based on the refractive index distribution. By thisstructure, the intensity distribution of the signal beam is the samebetween on the incident side and on the exit side.

Seventeenth Embodiment

Next, a seventeenth embodiment of the present invention will bedescribed. In the seventeenth embodiment, descriptions of the same partsas those of the fifteenth embodiment are omitted and only differentparts will be described. A multi-mode interference 1×2 splitter 5300 ofthe seventeenth embodiment has approximately the same structure as themulti-mode interference 1×2 splitter 5100 shown in FIG. 36, and isdifferent in the structure of an incident portion 5304, an exit portion5305 and a sheet-form optical transmission line 5301.

FIG. 40 is a cross-sectional view of a part, where the signal beam istransmitted, of the multi-mode interference 1×2 splitter 5300 accordingto the seventeenth embodiment of the present invention. FIG. 40 is across-sectional view of the multi-mode interference 1×2 splitter 5200taken on the same place as that in the case of the multi-modeinterference 1×2 splitter 5100 according to the fifteenth embodimentshown in FIG. 36 and FIG. 37.

The sheet-form optical transmission line 5301 has a refractive indexdistribution in the y-direction. The sheet-form optical transmissionline 5301 has the highest refractive index n_(max) at the centralportion 5101 a. The sheet-form optical transmission line 5301 has arefractive index distribution that satisfies Expression (1) with thecentral portion 5101 a as the symmetry plane. Moreover, the sheet-formoptical transmission line 5301 has a refractive index distribution onlyin the y-direction, and has no refractive index distribution in theother directions.

In the sheet-form optical transmission line 5301, the signal beamincident through the incident portion 5304 is condensed into a lineparallel to the x-direction at the central portion 5101 a of thesheet-form optical transmission line on the reflecting surface 5102.That is, by appropriately setting the length, in the y-direction, of theincident portion 5304, the signal beam can be condensed into a lineparallel to the x-direction at the central portion 5101 a of thesheet-form optical transmission line on the reflecting surface 5102.

Moreover, in the sheet-form optical transmission line 5301, the signalbeam exiting from the exit portion 5305 exits from the exit portion 5105after condensed into a line parallel to the x-direction at the centralportion 5101 a of the sheet-form optical transmission line on thereflecting surface 5103. That is, by appropriately setting the length,in the y-direction, of the exit portion 5305, the signal beam can becondensed into a line parallel to the x-direction at the central portion5101 a of the sheet-form optical transmission line on the reflectingsurface 5103.

Further, in the sheet-form optical transmission line 5301, at this time,the physical optical path length LR-R, at the central portion 5101 a,from the reflecting surface 5102 and the reflecting surface 5103 of thesheet-form optical transmission line 5301 satisfies the relationship ofthe following (Expression 17): $\begin{matrix}{L_{R - R} = {{\frac{2\pi}{g} \cdot \frac{j}{2}}\left( {{j = 0},1,2,3,\ldots} \right)}} & \left( {{Expression}\quad 17} \right)\end{matrix}$

(Expression 17) means that the physical optical path length LR-R isequal to a half-integral multiple of the meandering period of theoptical path of the signal beam that meanders in the sheet-form opticaltransmission line 5301. When the physical optical path length LR-R isset so as to satisfy (Expression 17), the signal beam condensed into aline parallel to the x-direction on the reflecting surface 5102 is againcondensed into a line parallel to the x-direction on the reflectingsurface 5103.

Therefore, between the reflecting surface 5101 and the reflectingsurface 5102, a conjugate relationship optically holds within a planeparallel to the y-z plane. At this time, since the refractive index thataffects the optical path A and the reflective index that affects theoptical path B completely coincide with each other, no phase differenceoccurs between the optical path A and the optical path B. As describedabove, the phase difference between the optical path A and the opticalpath B is zero when the physical optical path length LR-R of thesheet-form optical transmission line 5301 is set so as to satisfy(Expression 17).

As described above, in the optical device according to the seventeenthembodiment, of a plurality of optical paths transmitted through thesheet-form optical transmission line 5301, the difference in opticalpath length between the optical path A and the optical path B is zero.The phase difference between the optical path A and the optical path Bis the same between before the incidence on the optical transmissionline and after the exit from the optical transmission line.Consequently, in the optical device according to the seventeenthembodiment, the waveform at the time of the incidence on the opticaltransmission line and the waveform at the time of the exit therefrom canbe made to coincide with each other, so that the signal beam can be madeto exit from the optical transmission line without any loss.

Moreover, in the optical device according to the seventeenth embodiment,the optical path A and the optical path B do not have a part where anoptical path length difference is caused. By this structure, the phasedifference between the optical path A and the optical path B can be madezero.

Moreover, in the optical device according to the seventeenth embodiment,the above-described sheet-form optical transmission line includes thereflecting surface 5102 and the reflecting surface 5103, and thephysical optical path length between the reflecting surface 102 and thereflecting surface 103 at the central portion 101 a is equal to (j/2)times (j=0,1,2,3, . . . ) the period of meandering of the optical pathalong which the signal beam is transmitted while meandering based on therefractive index distribution. Moreover, in the optical device accordingto the seventeenth embodiment, the signal beam is condensed into a lineparallel to the x-direction orthogonal to both the y-direction and thez-direction at the central portion, where the thickness in the firstdirection is half, of the optical transmission line.

By this structure, the reflecting surfaces are optically in a conjugaterelationship at the central portion. For this reason, the two opticalpaths do not have a part where an optical path length difference iscaused, between the reflecting surfaces. Consequently, the phasedifference between the two optical paths can be made zero.

Eighteenth Embodiment

Next, an eighteenth embodiment of the present invention will bedescribed. In the eighteenth embodiment, descriptions of the same partsas those of the fifteenth embodiment are omitted and only differentparts will be described. A multi-mode interference 1×2 splitter 5400 ofthe eighteenth embodiment has approximately the same structure as themulti-mode interference 1×2 splitter 5100 shown in FIG. 36, and isdifferent in the structure of an incident portion 5404, an exit portion5405, an exit portion 5406 and a sheet-form optical transmission line5401.

FIG. 41A is a cross-sectional view of a part, where the signal beam istransmitted, of the multi-mode interference 1×2 splitter 5400 accordingto the eighteenth embodiment of the present invention. FIG. 41A is across-sectional view of the multi-mode interference 1×2 splitter 5400taken on the same place as that in the case of the multi-modeinterference 1×2 splitter 5100 according to the fifteenth embodimentshown in FIG. 36 and FIG. 37. In the figure, the refractive indexdistribution is omitted.

The sheet-form optical transmission line 5401 traps the externallyincident signal beam in the y-direction and can transmit it in thez-direction (transmission direction). The sheet-form opticaltransmission line 5401 has a reflecting surface 5402 and a reflectingsurface 5403 at both ends in the z-direction.

The incident portion 5404 is structured so that the optical axis of thesignal beam incident on the sheet-form optical transmission line 5401 isnot parallel to the z-direction but is at a predetermined acute anglewith respect thereto. Moreover, the exit portion 5405 is structured sothat the optical axis of the signal beam exiting from the sheet-formoptical transmission line 5401 is not parallel to the z-direction but isat a predetermined acute angle with respect thereto.

The reflecting surface 5402 is disposed so as to bend, in thez-direction, the optical axis of the signal beam transmitted through theincident portion 5405 and incident on the sheet-form opticaltransmission line 5401 from a direction at a predetermined acute anglewith respect to the z-direction. The reflecting surface 5403 is disposedso as to bend the signal beam in a direction in which the signal beam istransmitted through the sheet-form optical transmission line 5401 andthat is at a predetermined acute angle with respect to the z-direction.

In the sheet-form optical transmission line 5401, the signal beamincident through the incident portion 5404 is condensed into a lineparallel to the x-direction at the central portion 5101 a of thesheet-form optical transmission line on the reflecting surface 5402.That is, by appropriately setting the structure of the incident portion5404, the signal beam can be condensed into a line parallel to thex-direction at the central portion 5101 a of the sheet-form opticaltransmission line on the reflecting surface 5402.

Moreover, in the sheet-form optical transmission line 5401, the signalbeam exiting from the exit portion 5405 exits from the exit portion 5405after condensed into a line parallel to the x-direction at the centralportion 5101 a of the sheet-form optical transmission line on thereflecting surface 5403. That is, by appropriately setting the structureof the exit portion 5405, the signal beam can be condensed into a lineparallel to the x-direction at the central portion 5101 a of thesheet-form optical transmission line on the reflecting surface 5403.

As described above, in the multi-mode interference 1×2 splitter 5400according to the eighteenth embodiment, the optical axis of the signalbeam incident on the optical transmission line and the optical axis ofthe signal beam exiting from the optical transmission line are both notparallel to the z-direction but at the predetermined acute angle withrespect thereto. Consequently, the degree of freedom of the layout ofthe light emitting element 5111 and the light receiving element 5112 canbe improved.

In the eighteenth embodiment, it may be performed to calculate the phasedifference between the optical path A and the optical path B of thesignal beam in the sheet-form optical transmission line 5401 and makesthe phase difference a natural multiple of the signal beam wavelength orzero as described in the fifteenth embodiment and the sixteenthembodiment. By doing this, the phase difference between the optical pathA and the optical path B can also be made zero.

Nineteenth Embodiment

Next, a nineteenth embodiment of the present invention will bedescribed. In the nineteenth embodiment, descriptions of the same partsas those of the fifteenth embodiment are omitted and only differentparts will be described. A multi-mode interference 1×2 splitter 5500 ofthe nineteenth embodiment has approximately the same structure as themulti-mode interference 1×2 splitter 5100 shown in FIG. 36, and isdifferent in the structure of an incident portion 5504, an exit portion5505, an exit portion 5506 and a sheet-form optical transmission line5501.

FIG. 41B is a cross-sectional view of a part, where the signal beam istransmitted, of the multi-mode interference 1×2 splitter 5500 accordingto the nineteenth embodiment of the present invention. FIG. 41B is across-sectional view of the multi-mode interference 1×2 splitter 5500taken on the same place as that in the case of the multi-modeinterference 1×2 splitter 5100 according to the fifteenth embodimentshown in FIG. 1 and FIG. 2. In the figure, the refractive indexdistribution is omitted.

The sheet-form optical transmission line 5501 traps the externallyincident signal beam in the y-direction and can transmit it in thez-direction (transmission direction). The sheet-form opticaltransmission line 5501 has a reflecting surface 5502 and a reflectingsurface 5503 at both ends in the z-direction.

The incident portion 5504 is structured so that the optical axis of thesignal beam incident on the sheet-form optical transmission line 5401 isnot parallel to the z-direction but is at a predetermined acute anglewith respect thereto. Moreover, the exit portion 5505 is structured sothat the optical axis of the signal beam exiting from the sheet-formoptical transmission line 5501 is not parallel to the z-direction but isat a predetermined acute angle with respect thereto.

The reflecting surface 5502 is disposed so as to bend, in thez-direction, the optical axis of the signal beam transmitted through theincident portion 5504 and incident on the sheet-form opticaltransmission line 5501 from a direction at a predetermined acute anglewith respect to the z-direction. The reflecting surface 5503 is disposedso as to bend the signal beam in a direction in which the signal beam istransmitted through the sheet-form optical transmission line 5501 andthat is at a predetermined acute angle with respect to the z-direction.

In the sheet-form optical transmission line 5501, the signal beamincident through the incident portion 5504 is condensed into a lineparallel to the x-direction at the central portion 5101 a of thesheet-form optical transmission line on the reflecting surface 5502.That is, by appropriately setting the structure of the incident portion5504, the signal beam can be condensed into a line parallel to thex-direction at the central portion 5101 a of the sheet-form opticaltransmission line on the reflecting surface 5502.

Moreover, in the sheet-form optical transmission line 5501, the signalbeam exiting from the exit portion 5505 exits from the exit portion 5505after condensed into a line parallel to the x-direction at the centralportion 5101 a of the sheet-form optical transmission line on thereflecting surface 5503. That is, by appropriately setting the structureof the exit portion 5505, the signal beam can be condensed into a lineparallel to the x-direction at the central portion 5101 a of thesheet-form optical transmission line on the reflecting surface 5503.

At this time, the sheet-form optical transmission line 5501 satisfies(Expression 17) described in the seventeenth embodiment. Therefore,between the reflecting surface 5502 and the reflecting surface 5503, aconjugate relationship optically holds within a plane parallel to they-z plane. At this time, since the refractive index that affects theoptical path A and the reflective index that affects the optical path Bcompletely coincide with each other, no phase difference occurs betweenthe optical path A and the optical path B.

As described above, in the multi-mode interference 1×2 splitter 5500according to the nineteenth embodiment, the optical axis of the signalbeam incident on the optical transmission line and the optical axis ofthe signal beam exiting from the optical transmission line are both notparallel to the z-direction but at the predetermined acute angle withrespect thereto. Consequently, the degree of freedom of the layout ofthe light emitting element 5111 and the light receiving element 5112 canbe improved.

In the nineteenth embodiment, it may be performed to calculate the phasedifference between the optical path A and the optical path B of thesignal beam in the sheet-form optical transmission line 5501 and makesthe phase difference a natural multiple of the signal beam wavelength orzero as described in the fifteenth embodiment and the sixteenthembodiment. By doing this, the phase difference between the optical pathA and the optical path B can also be made zero.

Twentieth Embodiment

FIG. 42A is a cross-sectional view of a part, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter 5600 according toa twentieth embodiment of the present invention. In the twentiethembodiment, descriptions of the same parts as those of the fifteenthembodiment are omitted and only different parts will be described. Themulti-mode interference 1×2 splitter 5600 according to the twentiethembodiment has approximately the same structure as the multi-modeinterference 1×2 splitter 100 shown in FIG. 36, and is different only inthat a structure corresponding to the exit portion is not provided. Inthe figure, the refractive index distribution is omitted.

In FIG. 42A, the multi-mode interference 1×2 splitter 5600 according tothe twentieth embodiment is provided with an incident portion 5104 and asheet-form optical transmission line 5601. The structure of the incidentside of the sheet-form optical transmission line 5601 is the same asthat of the sheet-form optical transmission line 5101 according to thefirst embodiment. On the other hand, on the exit side, the signal beamexits in the z-direction from an end surface of the sheet-form opticaltransmission line 5601 from the exit side.

In the sheet-form optical transmission line 5601, the optical pathlength difference generating portion is only the reflecting surface5102. Therefore, by making the phase difference between the optical pathA and the optical path B caused at the reflecting surface 5102 anintegral multiple of the signal beam wavelength, the phase differencecan be made zero.

Twenty-First Embodiment

FIG. 42B is a cross-sectional view of a part, where the signal beam istransmitted, of a multi-mode interference 1×2 splitter 5700 according toa twenty-first embodiment of the present invention. In the twenty-firstembodiment, descriptions of the same parts as those of the fifteenthembodiment are omitted and only different parts will be described. Themulti-mode interference 1×2 splitter 5700 according to the twenty-firstembodiment has approximately the same structure as the multi-modeinterference 1×2 splitter 5100 shown in FIG. 1, and is different only inthat a structure corresponding to the incident portion is not provided.In the figure, the refractive index distribution is omitted.

In FIG. 42B, the multi-mode interference 1×2 splitter 5700 according tothe twenty-first embodiment is provided with an incident portion 5104and a sheet-form optical transmission line 5701. The structure of theexit side of the sheet-form optical transmission line 5701 is the sameas that of the sheet-form optical transmission line 5101 according tothe fifteenth embodiment. On the other hand, on the incident side, thesignal beam is incident in the z-direction through an end surface of thesheet-form optical transmission line 5701 from the incident side.

In the sheet-form optical transmission line 5701, the optical pathlength difference generating portion is only the reflecting surface5103. Therefore, by making the phase difference between the optical pathA and the optical path B caused at the reflecting surface 5103 anintegral multiple of the signal beam wavelength, the phase differencecan be made zero.

Twenty-Second Embodiment

Next, a twenty-second embodiment of the present invention will bedescribed with reference to FIG. 43A. In the twenty-second embodiment,descriptions of the same parts as those of the fifteenth embodiment areomitted and only different parts will be described. A multi-modeinterference 1×2 splitter 5800 of the twenty-second embodiment hasapproximately the same structure as the multi-mode interference 1×2splitter 5100 shown in FIG. 36, and is different in that a structurecorresponding to the exit portion is not provided and an intermediateincident and exit portion is present. In the figure, the refractiveindex distribution is omitted.

The multi-mode interference 1×2 splitter 5800 according to thetwenty-second embodiment is provided with an incident portion 5104, asheet-form optical transmission line 5801 and an intermediate incidentand exit portion 5820. The structure of the incident side of thesheet-form optical transmission line 5801 is the same as that of thesheet-form optical transmission line 5101 according to the firstembodiment. On the other hand, on the exit side, the signal beam exitsin the z-direction from an end surface of the sheet-form opticaltransmission line 5801 from the exit side.

The intermediate incident and exit portion 5820 includes a reflectingsurface 5813, an intermediate exit portion 5814, a processor 5816, anintermediate incident portion 5817 and a reflecting surface 5819.

The reflecting surface 5813 is a reflecting surface disposed at an angleof 5450 with respect to the z-x plane so as to bend the signal beamtransmitted in the positive direction of the z-direction, in thenegative direction of the y-direction.

The intermediate exit portion 5814 is a homogeneous-material prismhaving a triangle pole shape that extends in the x-direction. Theintermediate exit portion 5814 has a reflecting surface 5815 that bends,in the positive direction of the z-direction, the optical axis of thesignal beam bent in the negative direction of the y-direction.

The processor 5816 processes the incident signal beam with an opticalfilter. For example, the processor 5816 is a polarizing filter, ahalf-wave plate, a quarter-wave plate or an ND filter. Moreover, aliquid crystal element may be disposed. In this case, the processor 5816functions as an optical switch.

The intermediate incident portion 5817 is a homogeneous-material prismhaving a triangle pole shape that extends in the x-direction. Theintermediate incident portion 5817 has a reflecting surface 5818 thatbends, in the positive direction of the y-direction, the signal beamtransmitted in the z-direction.

The reflecting surface 5819 is a reflecting surface disposed at an angleof 45° with respect to the z-x plane so as to bend the signal beamincident in the positive direction of the y-direction, in the positivedirection of the z-direction.

In the above structure, the signal beam oscillated from the lightemitting point 5110 a is transmitted similarly to that in the fifteenthembodiment to reach the reflecting surface 5813. The optical axis of thesignal beam is bent in the negative direction of the y-direction by thereflecting surface 5813. Further, the signal beam is reflected at thereflecting surface 5815 of the intermediate exit portion 5814 to be bentin the positive direction of the z-direction. The optical axis of thesignal beam bent in the positive direction of the z-direction undergoespredetermined processing at the processor 5816, and is reflected at thereflecting surface 5818 of the intermediate incident portion 5817 to bebent in the positive direction of the y-direction. The optical axis ofthe signal beam bent in the positive direction of the y-direction isbent in the positive direction of the z-direction by the reflectingsurface 5819. The signal beam exits from an end of the sheet-formoptical transmission line in the end.

In the sheet-form optical transmission line 5801, the optical pathlength difference generating portions are the following three surfaces:the reflecting surface 5102, the reflecting surface 5813 and thereflecting surface 5819. Therefore, by making the sum of the differencesin optical path length between the optical path A and the optical path Bcaused at the three reflecting surfaces an integral multiple of thesignal beam wavelength, the overall phase difference between the opticalpath A and the optical path B can be made zero.

Moreover, by making zero the sum of the differences in optical pathlength between the optical path A and the optical path B caused at thethree reflecting surfaces, the overall phase difference between theoptical path A and the optical path B can be made zero.

Twenty-Third Embodiment

Next, a twenty-third embodiment of the present invention will bedescribed with reference to FIG. 43B. In the twenty-third embodiment,descriptions of the same parts as those of the fifteenth embodiment andthe twenty-second embodiment are omitted and only different parts willbe described. A multi-mode interference 1×2 splitter 5900 according tothe twenty-third embodiment has approximately the same structure as themulti-mode interference 1×2 splitter 5800 shown in FIG. 43A, and isdifferent in that the exit portion 5105 is the same as that of thefifteenth embodiment. In the figure, the refractive index distributionis omitted.

In the sheet-form optical transmission line 5901, the optical pathlength difference generating portions are the following four surfaces:the reflecting surface 5103, the reflecting surface 5813, the reflectingsurface 5819 and the reflecting surface 5103. Therefore, by making thesum of the differences in optical path length between the optical path Aand the optical path B caused at the four reflecting surfaces anintegral multiple of the signal beam wavelength, the overall phasedifference between the optical path A and the optical path B can be madezero.

Moreover, by making zero the sum of the differences in optical pathlength between the optical path A and the optical path B caused at thefour reflecting surfaces, the overall phase difference between theoptical path A and the optical path B can be made zero.

Other Embodiments

The fifteenth to twenty-third embodiments are not limited to theabove-described ones but may be appropriately modified. For example,while the part that causes the phase difference is the reflectingsurface in the embodiments, it may be a different structure as long asthe optical path length difference can be obtained.

While the light emitting element 5110 is a vertical cavity surfaceemitting laser in the embodiments, it may be a different element such asan edge emitting type laser. Moreover, the exit portion of a differentoptical transmission line that transmits the signal beam oscillated froman external light source may be disposed in the position of the lightemitting point 5110 a of the light emitting element 5110.

Moreover, while the light receiving element 5111 and the light receivingelement 5112 are photodiodes in the embodiments, they may be differentelements such as phototransistors. Moreover, the incident portion of adifferent optical transmission line for transmitting a signal beam maybe disposed in the positions of the light receiving point 5112 a of thelight receiving element 5111 and the light receiving point 5112 a of thelight receiving element.

While the refractive index distribution is a refractive indexdistribution such that the central refractive index is the highest(highest refractive index n_(max)) and the refractive index continuouslyand centrosymmetrically decreases with distance from the center towardthe periphery in the embodiments, the present invention is not limitedthereto. For example, it may be a refractive index distribution suchthat the refractive index stepwisely decreases from the center like astep function.

Further, while in the embodiments, the refractive index is uniform inthe directions, other than the y-direction, of the sheet-form opticaltransmission line to cause the multi-mode interference, in the case ofan optical data bus sheet or the like, a desired refractive indexdistribution may be provided to trap the signal beam in the x-direction.Moreover, the number of optical path length difference generatingportions where the optical path length difference is caused may be anarbitrary number.

For example, a structure may be adopted such that a number, m (m=1,2,3,. . . ), of optical path length difference generating portions where theoptical path length difference is caused are included and the sum of theoptical path length differences caused in the number, m, of optical pathlength difference generating portions is equal to a natural multiple ofthe signal beam wavelength.

Moreover, a structure may be adopted such that a number, n (n=2,3,4, . .. ), of optical path length difference generating portions where theoptical path length difference is caused are included and the sum of theoptical path length differences caused in the number, n, of optical pathlength difference generating portions is zero.

Moreover, while a parallel beam is incident on the sheet-form opticaltransmission line through the incident portion in the first and secondembodiments, a divergent beam or a convergent beam may be incident, andit is necessary only that symmetry of the signal beam with respect tothe central portion of the sheet-form optical transmission line bepresent.

As described above, the optical devices of the embodiments have thefollowing structures:

-   -   (1) An optical transmission line is provided that includes a        refractive index distribution in a first direction (the        y-direction in the above description) and is capable of        transmitting a signal beam in a second direction (the        z-direction in the above description) orthogonal to the first        direction, along a plurality of optical paths.

(2) At least one of the optical axis of the signal beam incident on theoptical transmission line and the optical axis of the signal beamexiting from the optical transmission line is not parallel to the seconddirection.

(3) The phase difference, at the time of the incidence on the opticaltransmission line, between the two optical paths, of the plurality ofoptical paths, incident on the optical transmission line symmetricallyto each other with respect to the optical axis of the signal beam andthe phase difference, at the time of the exit from the opticaltransmission line, between the two optical paths are the same.

Since the optical devices according to the embodiments have thestructure of (1), the optical transmission line is provided with arefractive index distribution, so that the mode dispersion is suppressedin the signal beam transmitted through the optical transmission line.Consequently, a collapse of the waveform of the transmitted signal beamdoes not occur, so that gigabit-class high-frequency signal beams can betransmitted in multiple modes.

Since the optical devices according to the embodiments have thestructure of (2), the incident portion and the exit portion function asnonparallel incident portions. Therefore, when optical parts such as alaser and a senor are mounted, it is easy to adjust the height betweenthe optical parts and the optical transmission line. Consequently, theseoptical parts can be easily mounted. Moreover, since optical parts canbe directly mounted on the electric purpose substrate, the opticaldevice can be made compact.

Moreover, since the optical devices according to the embodiments havethe structure of (3), the light beams transmitted along two opticalpaths are coupled together without any loss when exiting from theoptical transmission line.

In particular, when at least one of the optical axis of the signal beamincident on the optical transmission line and the optical axis of thesignal beam exiting from the optical transmission line is orthogonal tothe second direction like in the optical devices according to theembodiments, the outside and the optical transmission line can be easilycoupled together. For example, when optical parts such as a lightemitting element that emits the signal beam that is incident on theoptical transmission line and a light receiving element that receivesthe signal beam having exited from the optical transmission line arecoupled to the optical transmission line, the optical parts can beeasily mounted.

Self-Imaging Principle of the Multi-Mode Interference

Next, the relationship between the structure that compensates for theoptical path length difference described in the embodiments and thephysical optical path length of the sheet-form optical transmission linenecessary for splitting the signal beam based on the self-imagingprinciple of the multi-mode interference will be described. In thisdescription, for the sake of simplification, a case where one beam issplit into two beams by use of the self-imaging principle in the opticaldevice of the fifteenth embodiment will be described as an example.

FIG. 44 is a perspective view showing the structure of the multi-modeinterference 1×2 splitter 5100 according to the fifteenth embodiment ofthe present invention. FIG. 45 is a partial cross-sectional view of thesheet-form optical transmission line 5101 according to the fifteenthembodiment. FIG. 45 shows the C-D-G-H plane shown in FIG. 44. In FIGS.44 and 45, the detailed structure which is described in the fifteenthembodiment is omitted, and only parts required to be newly describedwill be described.

In FIGS. 44 and 45, the physical optical path length in the z-directionbased on the self-imaging principle is L1. Moreover, the physicaloptical path length from the reflecting surface 5102 to the reflectingsurface 5103 of the sheet-form optical transmission line 5101 at thecentral portion 5101 a is L2.

In FIG. 45, in the sheet-form optical transmission line 5101, therefractive index is uniform within a plane parallel to the z-x plane.Therefore, of the optical paths of the signal beam, the optical path Oincident on the central portion 5101 a travels in a straight linewithout affected by the refractive index distribution. The refractiveindex that affects the optical path O while the optical path O istraveling in a straight line is uniform. On the other hand, on theoptical path A, the refractive index incessantly changes as the beamtravels in the transmission direction. Therefore, in the description ofthe multi-mode interference, these two optical paths will be separatelydescribed.

The signal beam transmitted along the optical path O incident on thecentral portion 5101 a is transmitted within a plane of the uniformeffective refractive index n₀. Therefore, L1 can be calculated byapplying the self-imaging principle. According to the self-imagingprinciple, it is known that the configuration of the signal beamperiodically transmitted with the L_(π) shown in the following(Expression 18) as the unit returns to the same configuration as that ofthe incident signal beam. $\begin{matrix}{L_{\pi} = \frac{\pi}{\beta_{0} - \beta_{1}}} & \left( {{Expression}\quad 18} \right)\end{matrix}$

On the behavior of the incident signal beam, the calculation based onthe self-imaging principle can be performed according to the position,in the x-direction, where the signal beam is incident on the opticaltransmission line. For example, it is known that like in the fifteenthembodiment, for the signal beam incident on the central position in thex-direction, the same waveform is obtained with ¾L_(π) as the period.$\begin{matrix}{\frac{\pi}{\beta_{0} - \beta_{1}} = \frac{4\quad n_{0}W^{2}}{3\lambda}} & \left( {{Expression}\quad 19} \right)\end{matrix}$

Here, n₀ is the effective refractive index, of the 0th-order mode beamexcited in the direction of the width, corresponding to the highestrefractive index n_(max) at the center, W is the size, in thex-direction, of the sheet-form optical transmission line, and x is thewavelength of the transmitted signal beam.

As described above, the self-imaging principle is characterized in thatthe transmission line length of the sheet-form optical transmission lineis a function of the difference between the basic mode and the primarymode. Moreover, the self-imaging principle is characterized in that thedifference between the basic mode and the primary mode is approximatelydetermined by the wavelength λ of the signal beam, the effectiverefractive index no and the size W in the direction of the width.

Moreover, according to the self-imaging principle, the position wherethe signal beam incident on the central position in the x-directionpasses through the central position in the x-direction and is condensedso as to be split into a number, N, of beams symmetrically with respectto a plane parallel to the y-z plane is a position shifted by (1/N) ¾Lπin the z-direction from a position where the output waveform becomes thesame.

From the above, in order that the signal beam incident on the centralposition of the size W, in the x-direction, of the sheet-form opticaltransmission line 5101 passes through the central position in thex-direction and is condensed so as to be split into a number, N, ofbeams symmetrically with respect to a plane parallel to the y-z plane,it is necessary that the physical optical path length L1 satisfy thefollowing (Expression 20): $\begin{matrix}{L_{1} = {{{p*\frac{4}{3}L_{\pi}} \pm {\frac{1}{N}*\frac{4}{3}L_{\pi}}} = {{\left( {p \pm \frac{1}{N}} \right)\frac{4}{3}L_{\pi}} = {\left( {p \pm \frac{1}{N}} \right)\frac{n_{0}W^{2}}{\lambda}}}}} & \left( {{Expression}\quad 20} \right)\end{matrix}$

Here, since p (P≧0) and N (N≧1) are both integers and L1 is a positivenumber, a relationship where (p±1/N) is a positive number is satisfied.

Calculating L1 when the number of splits is two (when N=2), (Expression20) is modified to obtain the following (Expression 21): $\begin{matrix}{L_{1} = {{\left( {p \pm \frac{1}{2}} \right)\frac{n_{0}W^{2}}{\lambda}} = {\left( {{2\quad p} + 1} \right)\frac{n_{0}W^{2}}{2\lambda}\left( {{p = 0},1,2,\ldots} \right)}}} & \left( {{Expression}\quad 21} \right)\end{matrix}$

As is apparent from (Expression 21), in the case of the fifteenthembodiment, the signal beam can be split by providing the exit portionin a position corresponding to the period which is an odd multiple (1,3, 5, . . . ) of n₀W²/(2%) with n₀W²/(2λ) as the unit.

On the other hand, the optical path A incident on a position away fromthe center is transmitted while meandering in the sheet-form opticaltransmission line 5101 by being affected by the refractive indexdistribution. Therefore, the refractive index that affects the signalbeam transmitted along the optical path A is not uniform.

On the optical path A, when traveling in a direction away from thecentral portion 5101 a, the signal beam travels in a direction thatgradually increases the angle from the y-direction, because it alwaystravels from where the refractive index is high to where the refractiveindex is low. That is, when traveling in a direction away from thecentral portion 5101 a, the optical path A approaches so as to becomeparallel to the z-direction with distance from the central portion 5101a.

Conversely, on the optical path A, when traveling in a direction thatapproaches the central portion 5101 a, the signal beam travels in adirection that gradually decreases the angle from the y-direction,because it always travels from where the refractive index is low towhere the refractive index is low. That is, when traveling in adirection that approaches the central portion 5101 a, the optical path Aapproaches so as to become vertical to the z-direction as it approachesthe central portion 5101 a. By repeating this action, the optical path Atravels while meandering.

As described above, the signal beam on the optical path A always travelsat a finite angle with respect to the y-direction. Consequently, thespeed of the signal beam transmitted along the optical path A increaseswhen the signal beam travels in a direction away from the centralportion 5101 a. Conversely, the speed of the signal beam transmittedalong the optical path A decreases when the signal beam travels in adirection that approaches the central portion 5101 a.

The refractive index distribution of the sheet-form optical transmissionline 5101 is expressed by a quadratic function that satisfies theabove-mentioned (Expression 9). By appropriately setting the refractiveindex distribution, the speed component, in the z-direction, of theoptical path A is the same as the speed component, in the z-direction,of the optical path O.

That the speed component of the beam in the z-direction is constantmeans that there is no phase disturbance in the z-direction. Therefore,when the sheet-form optical transmission line 5101 is structured so asto satisfy (Expression 21), the signal beam transmitted along theoptical path A behaves similarly to the signal beam transmitted alongthe optical path O.

As described above, as long as the physical optical path length L1satisfies (Expression 21), the signal beam transmitted along the opticalpath A is condensed so as to be split into two beams in the x-directionbased on the self-imaging principle. For this reason, by providing theexit portion in the condensation position calculated based on(Expression 21), the signal beam can be made to exit so as to be splitinto two beams.

However, in order that the intensity distributions of the incidentsignal beam and the exiting signal beam completely match with eachother, it is necessary that the previously-described phase differencescaused at the reflecting surface 5102 and the reflecting surface 5103 bezero.

The physical optical path length of the sheet-form optical transmissionline 5101 where the phase difference is zero is as obtained by(Expression 12). Obtaining L2 from the condition of (Expression 12), thefollowing (Expression 22) which is (Expression 12) to which d is addedis obtained: $\begin{matrix}{L_{2} = {{\frac{2\pi}{g}j} + {d\quad\left( {{j = 0},1,2,3,\ldots} \right)}}} & \left( {{Expression}\quad 22} \right)\end{matrix}$

Table 2 shows results of concrete calculations of the relationshipbetween the shortest physical optical path length L1 where the signalbeam can be split into two beams based on the self-imaging principle ofthe multi-mode interference and the shortest physical optical pathlength L2 that compensates for the phase differences caused at thereflecting surfaces in the sheet-form optical transmission line 5101.Here, the refractive index n₀=1.5, the wavelength λ=0.85 μm, and therefractive index distribution coefficient g is set so as to decrease byapproximately 1% from the center at d/2. Moreover, of the L1 cells, 1×and 2×show the separation widths W, in the x-direction, of thesheet-form optical transmission line which widths W are once and twicethe thickness d, in the y-direction, of the sheet-form opticaltransmission line 5101 in the exit portion. TABLE 2 Thickness, in they-direction, of the sheet-form optical transmission line d [μm] 50 100200 1000 Refractive index 5.6 2.8 1.414 0.28 distribution coefficient g[mm⁻¹] Shortest physical 1.17 2.34 4.69 23.2 optical path length L2 thatcompensates for the phase differences caused at the reflecting surfacesL2 [mm] Shortest physical 1× 8.8 35.3 141 352 optical path length 2×35.2 141 565 14118 L1 where the signal beam can be split into two beamsby the multi-mode interference L1 [mm]

As is apparent from Table 2, to simultaneously satisfy the optical pathlength L2 and the optical path length L1, a value that is an integralmultiple of L2 and an odd multiple of L1 is adopted in the sheet-formoptical transmission line 5101.

However, it is difficult that these completely coincide with each other.Therefore, it is necessary to make a fine adjustment by use of L1 wherethe permissible width for the length in the transmission direction islarger than L2. As is understood from Table 2, since L2 takes a muchlower value than L1, in order that these match with each other, L2 isadjusted and a physical optical path length substantially coincidingwith L1 is adopted. For example, by multiplying L2 substantially by 8,L1 necessary for splitting only the width d into two beams in thex-direction is obtained.

As described above, in the optical devices of the embodiments, thephysical optical path length that compensates for the phase differenceof the signal beam incident from a direction orthogonal to therefractive index distribution and the physical optical path length wherethe signal beam can be split based on the self-imaging principle underthe condition of the self-imaging principle of the multi-modeinterference coincide with each other. Consequently, when the incidentsignal beam exits so as to be split into two beams by the multi-modeinterference, the incident and exit portions and the sheet-form opticaltransmission line can be coupled together without any loss.

As described above, in the optical devices of the embodiments, thephysical optical path length that compensates for the phase differencescaused at the reflecting surfaces and the physical optical path lengthbased on the condition of the self-imaging principle of the multi-modeinterference coincide with each other. Consequently, when the incidentsignal beam exits so as to be split into two beams by the multi-modeinterference, the incident and exit portions and the sheet-form opticaltransmission line can be coupled together without any loss.

While an example in which the signal beam is split into two beams insymmetrical positions, in the x-direction, of the sheet-form opticaltransmission line 5101 is shown in the above example, the presentinvention is not limited to the embodiment.

FIGS. 46A to 46D are schematic views showing examples of the input andoutput structure of the optical device. FIGS. 46A to 46D are all viewsviewed from a plane parallel to the z-x plane of the optical device.

FIG. 46A is a schematic view of an optical device, using the multi-modeinterference, of another embodiment. The optical device described inFIG. 46A is a splitter where the signal beam incident through oneincident portion Input 1 is split by the multi-mode interference andexits from two exit portions Output 1 and Output 2. The incident portionInput 1 is disposed in a position away from the central position, in thez-direction, of the optical device. The exit portion Output 1 isdisposed on a line passing through the incident portion Input 1 andparallel to the z-axis. The exit portion Output 2 is disposed at adistance in the x-direction from the exit portion Output 1.

FIG. 46B is a schematic view of an optical device, using the multi-modeinterference, of another embodiment. The optical device described inFIG. 46B is a combiner in which the signal beams incident through twoincident portions Input 1 and Input 2 are combined together by themulti-mode interference and exit from one exit portion Output 1. The twoinput portions Input 1 and Input 2 are disposed symmetrically withrespect to a line passing through the center in the x-direction andparallel to the z-axis. The exit portion Output 1 is disposed on theline passing through the center in the x-direction and parallel to thez-axis.

FIG. 46C is a schematic view of an optical device, using the multi-modeinterference, of another embodiment. The optical device described inFIG. 46C is a combiner in which the signal beams incident through twoincident portions Input 1 and Input 2 are combined together by themulti-mode interference and exit from one exit portion Output 1. The twoinput portions Input 1 and Input 2 are disposed symmetrically withrespect to the line passing through the center in the x-direction andparallel to the z-axis. The exit portion Output 1 is disposed on theline passing through the input portion Input 1 and parallel to thez-axis.

FIG. 46D is a schematic view of an optical device, using the multi-modeinterference, of another embodiment. The optical device described inFIG. 46D is a combiner in which the signal beams incident through twoincident portions Input 1 and Input 2 are combined together and split bythe multi-mode interference and exit from two exit portions Output 1 andOutput 2. The two input portions Input 1 and Input 2 are disposedsymmetrically with respect to the line passing through the center in thex-direction and parallel to the z-axis. The two exit portions Output 1and Output 2 are disposed symmetrically with respect to the line passingthrough the center in the x-direction and parallel to the z-axis.

The condition of the self-imaging principle of the multi-modeinterference differs among the optical devices described in FIGS. 46A to46D. Therefore, the condition of the self-imaging principle of themulti-mode interference is changed and the physical optical path lengthis adjusted. By doing this, in the optical devices described in FIGS.46A to 46D, the physical optical path length that compensates for thephase difference caused at the reflecting surfaces and the physicaloptical path length based on the condition of the self-imaging principleof the multi-mode interference coincide with each other. Consequently,when the incident signal beam is split by the multi-mode interferenceand made to exit, the incident and exit portions and the sheet-formoptical transmission line can be coupled together without any loss.

Further, the present invention is not limited to the above-describedincident and exit structure of the optical device, but is applicable toan optical device that has a number, M (M=1,2,3, . . . ), of incidentportions for making a signal beam incident on the sheet-form opticaltransmission line and a number, N (N=1,2,3, . . . ), of exit portionsfor making a signal beam exit from the sheet-form optical transmissionline, and couples the incident and exit portions by the multi-modeinterference.

(1) A sheet-form optical transmission line including a refractive indexdistribution in a first direction, being capable of transmitting thesignal beam in a second direction orthogonal to the first direction, andbeing capable of trapping the signal beam in the first direction,

(2) a number, M (M=1,2,3, . . . ), of incident portions for making thesignal beam incident on the optical transmission line,

(3) a number, N (N=1,2,3, . . . ), of exit portions for making thesignal beam exit from the optical transmission line are provided, and

(4) the number, M, of incident portions and the number, N, of exitportions include at least one nonparallel incident and exit portion thatis coupled to the sheet-form optical transmission line in a directionwhere the optical axis of the signal beam transmitted inside is notparallel to the second direction.

(5) Between two optical paths incident on the sheet-form opticaltransmission line symmetrically to each other with respect to theoptical axis of the signal beam, of a plurality of optical paths of thesignal beam transmitted between the nonparallel incident and exitportion and the corresponding incident portion or exit portion, thephase difference at the time of the incidence on the sheet-form opticaltransmission line and the phase difference at the time of the exit fromthe sheet-form optical transmission line are the same.

(6) The number, M, of incident portions and the number, N, of exitportions are all disposed in positions satisfying the condition of theself-imaging principle of the multi-mode interference.

Since the multi-mode interference 1×2 splitter according to theembodiment has the structure of (1), the mode dispersion is suppressedin the signal beam transmitted through the optical transmission line.Consequently, a collapse of the waveform of the transmitted signal beamdoes not occur, so that gigabit-class high-frequency signal beams can betransmitted in multiple modes.

Since the multi-mode interference 1×2 splitter according to theembodiment has the structure of (4), when optical parts such as a laserand a senor are mounted, it is easy to adjust the height between theoptical parts and the optical transmission line. Consequently, theseoptical parts can be easily mounted. Moreover, since optical parts canbe directly mounted on the electric purpose substrate, the opticaldevice can be made compact.

Moreover, since the multi-mode interference 1×2 splitter according tothe embodiment has the structure of (5), the light beams transmittedalong two optical paths are coupled together without any loss whenexiting from the optical transmission line.

Moreover, since the multi-mode interference 1×2 splitter according tothe embodiment has the structures of (2), (3) and (6), the signal beamsincident through the incident portions exit from the exit portions bythe multi-mode interference.

Twenty-Fourth Embodiment

FIG. 47A is a perspective view showing the general outline of a gradedindex slab waveguide of an optical device that splits one beam into twobeams according to a twenty-fourth embodiment of the present invention.The optical device according to the twenty-fourth embodiment comprisesas a main element a graded index slab waveguide 4701 that transmitsbeams as shown in FIG. 47A. The graded index slab waveguide 4701 is, asshown in FIG. 47A, a sheet-form multi-mode optical transmission linethat extends on the x-z plane. The graded index slab waveguide 4701 hasa refractive index distribution such that the highest refractive indexn_(max) is provided at the center in the direction of the thickness andthe refractive index does not increase with distance from the center.The graded index slab waveguide 4701 has a uniform refractive index inthe direction of the width and has no refractive index. The graded indexslab waveguide 4701 has an incident and exit surface 4702 and areflecting surface 4703.

The incident and exit surface 4702 is opposed to all of an incidentportion (not shown) that makes an incident beam 4704 incident on thecentral position in the direction of the width, a light receivingportion (not shown) that receives an exiting beam 4705 exiting from aposition symmetrical with respect to the center in the direction of thewidth and a light receiving portion (not shown) that receives an exitingbeam 4706. Moreover, the reflecting surface 4703 is a total reflectionsurface formed by evaporating a metal such as aluminum onto an endsurface. The reflecting surface 4703 totally reflects all of theincident signal beam.

In the twenty-fourth embodiment, in the graded index slab waveguide4701, the slab length L/2 substantially coincides with n₀×W₀ ²/(4λ), andthe distance D1 between the exiting beam 4705 and the exiting beam 4706substantially coincides with W₀/2. Here, n0 is the effective refractiveindex of the 0th-order mode beam excited in the direction of the width.

The graded index slab waveguide 4701 has a slab length half that of thegraded index slab waveguide 101 described in the optical device thatsplits one beam incident on the central position into two beams whichoptical device is described in the first embodiment. Therefore, thesignal beam incident on the graded index slab waveguide 4701 through theincident portion propagates along a length equal to the optical pathdescribed in the first embodiment by propagating in the positivedirection of the z-direction, being reflected at the reflecting surface4703 and propagating in the negative direction of the z-direction.Consequently, the signal beam forms images having the same profile asthe incident beam in positions of the exiting beam 4.705 and the exitingbeam 4706 based on the self-imaging principle of the multi-modeinterference. The formed images are outputted as the exiting beams.

As described above, according to the optical device of the twenty-fourthembodiment, an optical splitter that splits an incident beam incident onthe central position in the direction of th width into two beamssymmetrically with respect to the center in the direction of the widthcan be realized with a graded index slab waveguide having a slab lengthhalf that of the first embodiment. Moreover, the optical deviceaccording to the twenty-fourth embodiment is capable of making theincident beam incident on the central position in the direction of thewidth exit from the same surface on which the incident beam is incident,as two exiting beams into which the incident beam is split symmetricallywith respect to the center in the direction of the width.

FIG. 47B is a perspective view showing the general outline of a gradedindex slab waveguide of an optical device that splits one beam into twobeams according to a modification of the twenty-fourth embodiment of thepresent invention. The optical device according to the modification ofthe twenty-fourth embodiment comprises as a main element a graded indexslab waveguide 4801 that transmits beams as shown in FIG. 47B. Thegraded index slab waveguide 4801 is, as shown in FIG. 47B, a sheet-formmulti-mode optical transmission line that extends on the x-z plane. Thegraded index slab waveguide 4801 has a refractive index distributionsuch that the highest refractive index n_(max) is provided at the centerin the direction of the thickness and the refractive index does notincrease with distance from the center. The graded index slab waveguide4801 has a uniform refractive index in the direction of the width andhas no refractive index. The graded index slab waveguide 4801 has anincident surface 4802, a reflecting surface 4803 and an exit surface4807.

The incident surface 4802 is opposed to all of an incident portion (notshown) that makes an incident beam 4804 incident on the central positionin the direction of the width, alight receiving portion (not shown) thatreceives an exiting beam 4805 exiting from a position symmetrical withrespect to the center in the direction of the width and a lightreceiving portion (not shown) that receives an exiting beam 4806. Theexit surface 4807 is opposed to all of a light receiving portion (notshown) that receives an exiting beam 4808 exiting from a positionsymmetrical with respect to the center in the direction of the width anda light receiving portion (not shown) that receives an exiting beam4809. Moreover, the reflecting surface 4803 is a half mirror formed byevaporating a metal such as aluminum onto an end surface. The reflectingsurface 4803 transmits half of the incident signal beam and reflects theremainder of the incident signal beam.

In the modification of the twenty-fourth embodiment, in the graded indexslab waveguide 4801, the slab length L substantially coincides withn₀×W₀ ²/(2×), and the distance D1 between the exiting beam 4805 and theexiting beam 4806 substantially coincides with W₀/2. Moreover, thereflecting surface 4803 is formed in a position where the slab length Lis just half the length. Here, no is the effective refractive index ofthe 0th-order mode beam excited in the direction of the width.

By the above structure, the signal beam incident on the graded indexslab waveguide 4801 through the incident portion propagates in thepositive direction of the z-direction, and part thereof is reflected atthe reflecting surface 4803 and the remainder thereof is transmitted.The reflected signal beam exits from the incident surface 4802 as theexiting beam 4805 and the exiting beam 4806 based on the self-imagingprinciple of the multi-mode interference as described in thetwenty-fourth embodiment. On the other hand, the transmitted signal beamexits as the exiting beam 4808 and the exiting beam 4809 based on theself-imaging principle of the multi-mode interference completelysimilarly to the first embodiment.

As described above, according to the optical device of the modificationof the twenty-fourth embodiment, an optical splitter that splits theincident beam incident on the central position in the direction of thewidth into four beams symmetrically with respect to the center in thedirection of the width can be realized with a graded index slabwaveguide having the same slab length as the first embodiment. Moreover,the optical device according to the modification of the twenty-fourthembodiment is capable of making the incident beam incident on thecentral position in the direction of the width exit in two differentdirections as two exiting beams into which the incident beam is splitsymmetrically with respect to the center in the direction of the width.

INDUSTRIAL APPLICABILITY

The present invention is suitable for optical devices such as an opticalsplitter, an optical combiner, an optical demultiplexer, an opticalmultiplexer, a star coupler and an optical switch used for high-speedmulti-mode optical communication. Moreover, the present invention issuitable for an optical straight sheet bus, an optical cross sheet busand the like used for high-speed multi-mode optical wiring.

1. An optical device that connects, by a signal beam, between anexternally inputted input signal and an output signal to be outputted,the optical device comprising: an optical transmission line beingsheet-form and including a refractive index distribution such that ahighest refractive index part is provided in a direction of a thicknessof the sheet and a refractive index does not increase with distance fromthe highest refractive index part in the direction of the thickness,wherein a signal beam corresponding to the input signal is made incidenton the optical transmission line as an incident beam, wherein inside theoptical transmission line, the incident beam is transmitted, in adirection of a length that is orthogonal to the direction of thethickness, in multiple modes having a plurality of eigenmodes in adirection of a width that is orthogonal to both the direction of thelength and the direction of the thickness, and an exiting beam isgenerated by the plurality of eigenmodes interfering with each other inthe direction of the length, and wherein the exiting beam is made toexit from the optical transmission line, and the output signalcorresponding to the exiting beam is outputted.
 2. An optical deviceaccording to claim 1, wherein the optical transmission line has a size,in the direction of the length, expressed by a function of a differencebetween a propagation constant of a 0th-order mode excited in thedirection of the width of the optical transmission line and apropagation constant of a primary mode.
 3. An optical device accordingto claim 1, wherein the optical transmission line has a size, in thedirection of the length, expressed by a function of a basic mode widthin the direction of the width, the highest refractive index in thedirection of the thickness and a wavelength of a beam transmitted in themulti-mode optical transmission line.
 4. An optical device according toclaim 1, wherein the optical transmission line includes a refractiveindex distribution such that a central position in the direction of thethickness has the highest refractive index and the refractive index doesnot increase with distance from the central position.
 5. An opticaldevice according to claim 4, wherein the refractive index distributionchanges substantially along a quadratic function.
 6. An optical deviceaccording to claim 4, wherein the optical transmission line is made ofpolysilane.
 7. An optical device according to claim 6, wherein theoptical transmission line is made of polysilane, and the refractiveindex distribution is provided by an oxygen concentration distributionwhen the polysilane is cured.
 8. An optical device according to claim 1,wherein the input signal is an electric signal, and an incident portionis provided that converts the electric signal into the signal beam andmakes the signal beam incident on the optical transmission line as theincident beam.
 9. An optical device according to claim 8, wherein theincident portion has a plurality of light emitting portions disposed inan array in the direction of the width of the optical transmission line.10. An optical device according to claim 1, wherein the input signal isa signal beam, and an incident portion is provided that makes the signalbeam incident on the optical transmission line as an incident beam. 11.An optical device according to claim 1, wherein the output signal is anelectric signal, and an exit portion is provided that receives thesignal beam as an exiting beam having exited from the opticaltransmission line and converts the signal beam into the electric signal.12. An optical device according to claim 11, wherein the exit portionhas a plurality of light receiving portions disposed in an array in thedirection of the width of the optical transmission line.
 13. An opticaldevice according to claim 1, wherein the output signal is a signal beam,and an exit portion is provided that makes the signal beam exit from theoptical transmission line as an exiting beam.
 14. An optical deviceaccording to claim 1, wherein the optical device is a 1×N opticalsplitting device that is capable of receiving at least one input signaland outputting the input signal as a number, N (N=1,2,3, . . . ), ofoutput signals, and wherein the optical transmission line includes: anincident surface for making the incident beam incident; and an exitsurface for making the exiting beam exit, the size in the direction ofthe length is a value that is substantially an integral multiple of thefollowing expression when the basic mode width in the direction of thewidth is W₀, an effective refractive index of a 0th-order mode beamexcited in the direction of the width is n₀ and the wavelength of thebeam transmitted in the multi-mode optical transmission line is λ, andone incident beam is made incident on a center in the direction of thewidth on the incident surface and a number, N, of exiting beams aregenerated symmetrically with respect to the center in the direction ofthe width on the exit surface:$\frac{1}{N} \cdot \frac{n_{0}W_{0}^{2}}{\lambda}$
 15. An optical deviceaccording to claim 1, wherein the optical device is an N×1 opticalcombining device that is capable of receiving a number, N (N=1,2,3, . .. ), of input signals and outputting the input signals as at least oneoutput signal, and wherein the optical transmission line includes: anincident surface for making the incident beam incident;, and an exitsurface for making the exiting beam exit, the size in the direction ofthe length is a value that is substantially an integral multiple of thefollowing expression when the basic mode width in the direction of thewidth is W₀, an effective refractive index of a 0th-order mode beamexcited in the direction of the width is n₀ and the wavelength of thebeam transmitted in the multi-mode optical transmission line is λ, and anumber, N, of incident beams all having the same wavelength λ are madeincident symmetrically with respect to a center in the direction of thewidth on the incident surface and one exiting beam is generated at thecenter in the direction of the width on the exit surface:$\frac{1}{N} \cdot \frac{n_{0}W_{0}^{2}}{\lambda}$
 16. An optical deviceaccording to claim 1, wherein the optical device is a straight sheet busthat is capable of receiving a number, N (N=1,2,3, . . . ), of inputsignals and outputting the input signals as a number, N, of outputsignals corresponding one-to-one to the input signals, and wherein theoptical transmission line includes: an incident surface for making theincident beam incident; and an exit surface for making the exiting beamexit, the size in the direction of the length is a value that issubstantially an integral multiple of the following expression when thebasic mode width in the direction of the width is W₀, an effectiverefractive index of a 0th-order mode beam excited in the direction ofthe width is n₀ and the wavelength of the beam transmitted in themulti-mode optical transmission line is λ, and a number, N, of incidentbeams all having the same wavelength λ are made incident on givenpositions in the direction of the width on the incident surface and anumber, N, of exiting beams corresponding one-to-one to the number, N,of incident beams are generated in positions, on the exit surface, whosepositions in the direction of the width are the same as incidentpositions of the incident beams: $\frac{8n_{0}W_{0}^{2}}{\lambda}$ 17.An optical device according to claim 1, wherein the optical device is across sheet bus that is capable of receiving a number, N (N=1,2,3, . . .), of input signals and outputting the input signals as a number, N, ofoutput signals corresponding one-to-one to the input signals, andwherein the optical transmission line includes: an incident surface formaking the incident beam incident; and an exit surface for making theexiting beam exit, a size in the direction of the length is a value thatis substantially an odd multiple of the following expression when thebasic mode width in the direction of the width is W₀, an effectiverefractive index of a 0th-order mode beam excited in the direction ofthe width is n₀ and the wavelength of the beam transmitted in themulti-mode optical transmission line is λ, and a number, N, of incidentbeams all having the same wavelength λ are made incident on givenpositions in the direction of the width on the incident surface and anumber, N, of exiting beams corresponding one-to-one to the number, N,of incident beams are generated in positions, on the exit surface, whosepositions in the direction of the width are symmetrical to incidentpositions of the incident beams with respect to the center in thedirection of the width: $\frac{4n_{0}W_{0}^{2}}{\lambda}$
 18. An opticaldevice according to claim 1, wherein the optical device is a starcoupler that receives a number, N (N=1,2,3, . . . ), of input signalsand outputs the input signals as a number, N, of output signalscorresponding to the input signals, and wherein the optical transmissionline includes: an incident surface for making the incident beamincident; and an exit surface for making the exiting beam exit, a sizein the direction of the length is substantially a value of the followingexpression when the basic mode width in the direction of the width isW₀, an effective refractive index of a 0th-order mode beam excited inthe direction of the width is n₀ and the wavelength of the beamtransmitted in the multi-mode optical transmission line is λ, and anumber, N, of incident beams all having the same wavelength λ are madeincident on predetermined positions in the direction of the width on theincident surface and a number, N, of exiting beams are generated for anyone of the incident beams in positions, on the exit surface, whosepositions in the direction of the width are symmetrical to incidentpositions of the incident beams with respect to the center in thedirection of the width:$\left( {p \pm \frac{1}{N}} \right)\frac{4n_{0}W_{0}^{2}}{\lambda}$ (pis an integer that makes the value inside the parentheses positive) 19.An optical device according to claim 18, wherein the optical device is astar coupler that receives a number, N_(EVEN) (N_(EVEN)=2,4,6, . . . ),of input signals and outputs the input signals as a number, N_(EVEN), ofoutput signals corresponding to the input signals, and wherein theoptical transmission line makes a number, N_(EVEN), of incident beamsall having the same wavelength λ incident on positions symmetrical withrespect to the center in the direction of the width on the incidentsurface.
 20. An optical device according to claim 18, wherein theoptical device is a star coupler that receives a number, N_(ODD)(N_(ODD)=1,3,5, . . . ), of input signals and outputs the input signalsas a number, N_(ODD), of output signals corresponding to the inputsignals, and wherein the optical transmission line makes a number,N_(ODD), of incident beams all having the same wavelength λ incident onpositions asymmetrical with respect to the center in the direction ofthe width on the incident surface.
 21. An optical device according toclaim 1, wherein the optical device is a two-way straight sheet bus thatis capable of receiving a number, N (N=1,2,3, . . . ), of input signalsand outputting the input signals as a number, N, of output signalscorresponding one-to-one to the first input signals, and is capable ofreceiving a number, M (M=1,2,3, . . . ), of input signals and outputtingthe input signals as a number, M, of output signals correspondingone-to-one to the input signals, and wherein the optical transmissionline includes: a first surface formed at one end in the direction of thelength; and a second surface formed at another end in the direction ofthe length, a size in the direction of the length is a value that issubstantially an integral multiple of the following expression when thebasic mode width in the direction of the width is W₀, an effectiverefractive index of a 0th-order mode beam excited in the direction ofthe width is no and the wavelength of the beam transmitted in themulti-mode optical transmission line is λ, a number, N, of incidentbeams all having the same wavelength λ are made incident on givenpositions in the direction of the width on the first surface and anumber, N, of exiting beams corresponding one-to-one to the number, N,of incident beams are generated in positions, on the second surface,whose positions in the direction of the width are the same as incidentpositions of the incident beams, and a number, M, of incident beams allhaving the same wavelength λ as the incident beams on the first surfaceare made incident on given positions in the direction of the width onthe second surface and a number, M, of exiting beams correspondingone-to-one to the number, M, of incident beams are generated inpositions, on the first surface, whose positions in the direction of thewidth are the same as incident positions of the incident beams:$\frac{8n_{0}W_{0}^{2}}{\lambda}$
 22. An optical device according toclaim 1, wherein the optical device is a two-way cross sheet bus that iscapable of receiving a number, N (N=1,2,3, . . . ), of first inputsignals and outputting the input signals as a number, N, of first outputsignals corresponding one-to-one to the first input signals, and iscapable of receiving a number, M (M=1,2,3, . . . ), of second inputsignals and outputting the input signals as a number, M, of outputsignals corresponding one-to-one to the second input signals, andwherein the optical transmission line includes: a first surface formedat one end in the direction of the length; and a second surface formedat another end in the direction of the length, a size in the directionof the length is a value that is substantially an odd multiple of thefollowing expression when the basic mode width in the direction of thewidth is W₀, an effective refractive index of a 0th-order mode beamexcited in the direction of the width is n₀ and the wavelength of thebeam transmitted in the multi-mode optical transmission line is λ, anumber, N, of incident beams all having the same wavelength λ are madeincident on given positions in the direction of the width on the firstsurface and a number, N, of exiting beams corresponding one-to-one tothe number, N, of incident beams are generated in positions, on thesecond surface, whose positions in the direction of the width aresymmetrical to incident positions of the incident beams with respect tothe center in the direction of the width, and a number, M, of incidentbeams all having the same wavelength λ are made incident on givenpositions in the direction of the width on the second surface and anumber, M, of exiting beams corresponding one-to-one to the number, M,of incident beams are generated in positions, on the first surface,whose positions in the direction of the width are symmetrical toincident positions of the incident beams with respect to the center inthe direction of the width: $\frac{4n_{0}W_{0}^{2}}{\lambda}$
 23. Anoptical device according to claim 1, wherein the optical transmissionline includes: a reflecting surface that is formed at one end in thedirection of the length and bends an optical path of the incident beamincident in a direction parallel to the direction of the thickness,substantially 90 degrees in the direction of the length; and/or areflecting surface that is formed at another end in the direction of thelength and bends an optical path of the exiting beam transmitted in thedirection of the length, substantially 90 degrees so as to exit in adirection parallel to the direction of the thickness.
 24. An opticaldevice according to claim 1, wherein the optical transmission lineincludes: a prism that is formed at one end in the direction of thelength and bends, in the direction of the length, an optical path of theincident beam incident in a direction inclined in the direction of thethickness; and/or a prism that is formed at another end in the directionof the length and bends an optical path of the exiting beam transmittedin the direction of the length, so as to exit in a direction inclined inthe direction of the thickness.
 25. An optical device according to claim1, wherein the optical transmission line has a plurality of eigenmodesin the direction of the thickness.
 26. An optical device according toclaim 1, wherein the optical transmission line has a thickness of notless than 20 μm.
 27. An optical device according to claim 1, wherein theoptical transmission line is curved so that a central position in thedirection of the thickness always draws the same curve on given twodifferent cross sections including the direction of the length and thedirection of the thickness.
 28. An optical device according to claim 1,wherein the optical transmission line is twisted so that a centralposition in the direction of the thickness draws different curves ongiven two different cross sections including the direction of the lengthand the direction of the thickness.
 29. An optical integrated devicethat connects, by a signal beam, between an externally inputted inputsignal and an output signal to be outputted, the optical integrateddevice comprising: a light transmitting portion comprising a pluralityof optical transmission lines being sheet-form and including arefractive index distribution such that a highest refractive index partis provided in a direction of a thickness of the sheet and a refractiveindex does not increase with distance from the highest refractive indexpart in the direction of the thickness, the optical transmission linesbeing laminated in the direction of the thickness, wherein a signal beamcorresponding to the input signal is made incident on the opticaltransmission lines as an incident beam, wherein inside the opticaltransmission lines, the incident beam is transmitted, in a direction ofa length that is orthogonal to the direction of the thickness, inmultiple modes having a plurality of eigenmodes in a direction of awidth that is orthogonal to both the direction of the length and thedirection of the thickness, and an exiting beam is generated by theplurality of eigenmodes interfering with each other in the direction ofthe length, and wherein the exiting beam is made to exit from theoptical transmission lines, and the output signal corresponding to theexiting beam is outputted.
 30. A method of manufacturing an opticaldevice that connects, by a signal beam, between an externally inputtedinput signal and an output signal to be outputted, wherein the opticaldevice comprises an optical transmission line being sheet-form andincluding a refractive index distribution such that a highest refractiveindex part is provided in a direction of a thickness of the sheet and arefractive index does not increase with distance from the highestrefractive index part in the direction of the thickness, wherein asignal beam corresponding to the input signal is made incident on theoptical transmission line as an incident beam, wherein inside theoptical transmission line, the incident beam is transmitted, in adirection of a length that is orthogonal to the direction of thethickness, in multiple modes having a plurality of eigenmodes in adirection of a width that is orthogonal to both the direction of thelength and the direction of the thickness, and an exiting beam isgenerated by the plurality of eigenmodes interfering with each other inthe direction of the length, wherein the exiting beam is made to exitfrom the optical transmission line, and the output signal correspondingto the exiting beam is outputted, and wherein the optical devicemanufacturing method comprises: a first step of preparing a forming diethat is made of a material capable of transmitting an energy to beapplied to cure a resin of which the optical transmission line is made,and includes a concave portion having at least the same depth as thedirection of the thickness of the optical transmission line; a secondstep of filling the concave portion with the resin; a third step ofapplying the energy in a predetermined quantity to the forming diefilled with the resin, from above and below in the direction of thethickness; and a fourth step of, on the resin cured with a desiredrefractive index distribution being formed, determining at least a sizein the direction of the length and forming a part of connection of theincident and exiting beams in order to form the resin into the opticaltransmission line.
 31. An optical device manufacturing method accordingto claim 30, wherein in the third step, the application of the energy isan application of an ultraviolet ray of a predetermined wavelength, andwherein in the first step, the prepared forming die is made of amaterial that is transparent with respect to the ultraviolet ray of thepredetermined wavelength.
 32. An optical device manufacturing methodaccording to claim 30, wherein in the third step, the application of theenergy is heating.
 33. An optical device manufacturing method accordingto claim 30, wherein the optical transmission line includes a refractiveindex distribution such that a central position in the direction of thethickness has the highest refractive index and the refractive index doesnot increase with distance from the central position.
 34. An opticaldevice manufacturing method according to claim 33, wherein therefractive index distribution changes substantially along a quadraticfunction.
 35. An optical device manufacturing method according to claim33, wherein the optical transmission line is made of polysilane.
 36. Anoptical device manufacturing method according to claim 35, wherein theoptical transmission line is made of polysilane, and the refractiveindex distribution is provided by an oxygen concentration distributionwhen the polysilane is cured.
 37. An optical device manufacturing methodaccording to claim 30, wherein in the first step, the forming dieincludes a concave portion having a size including a plurality ofoptical transmission lines to be manufactured, and wherein in the fourthstep, a plurality of optical transmission lines is simultaneouslymanufactured by cutting the resin.
 38. An optical device manufacturingmethod according to claim 30, wherein in the first step, the forming dieincludes a concave portion having a size substantially equal to a size,in the direction of the width, of the optical transmission line to bemanufactured, and wherein in the fourth step, the size in the directionof the length is determined by cutting the resin.
 39. An optical devicemanufacturing method according to claim 30, wherein in the first step,the forming die includes a concave portion having a size substantiallyequal to a size of the optical transmission line to be manufactured, andwherein in the fourth step, a wall, of the concave portion, situated ina position where the incident beam and the exiting beam are madeincident and made to exit on and from the optical transmission line isremoved.
 40. An optical device manufacturing method according to claim30, further comprising a fifth step of releasing the opticaltransmission line from the forming die either before or after the fourthstep.
 41. An optical device that is capable of receiving a multiplesignal beam where two different wavelengths are superimposed on eachother, demultiplexing the multiple signal beam according to thewavelength, and outputting the multiple signal beam as two differentsignal beams, the optical device comprising: an optical transmissionline being sheet-form and including a refractive index distribution suchthat a highest refractive index part is provided in a direction of athickness of the sheet and a refractive index does not increase withdistance from the highest refractive index part in the direction of thethickness, wherein the multiple signal beam is made incident on theoptical transmission line as an incident beam, wherein inside theoptical transmission line, the incident beam is transmitted, in adirection of a length that is orthogonal to the direction of thethickness, in multiple modes having a plurality of eigenmodes for eachwavelength in a direction of a width that is orthogonal to both thedirection of the length and the direction of the thickness, and twoexiting beams are generated in different positions in the direction ofthe width according to the wavelength by the plurality of eigenmodesinterfering with each other in the direction of the length with respectto signal beams of the same wavelength, and wherein the two exitingbeams are made to exit from the optical transmission line.
 42. Anoptical device according to claim 41, wherein the two exiting beams aremade to exit from positions in the direction of the width where a ratioin light quantity between the two exiting beams is highest.
 43. Anoptical device according to claim 41, wherein the two exiting beams aremade to exit from positions in the direction of the width where lightquantities of the two exiting beams are lowest.
 44. An optical deviceaccording to claim 41, wherein the optical transmission line has a sizein the direction of the length expressed by a function of a differencebetween a propagation constant of a 0th-order mode excited in thedirection of the width of the optical transmission line and apropagation constant of a primary mode.
 45. An optical device accordingto claim 41, wherein the optical transmission line has a rectangularparallelepiped shape, and has a size in the direction of the lengthexpressed by a function of a basic mode width in the direction of thewidth, the highest refractive index in the direction of the thicknessand a wavelength of a beam transmitted in the multi-mode opticaltransmission line.
 46. An optical device according to claim 41, whereinthe optical transmission line includes a refractive index distributionsuch that a central position in the direction of the thickness has thehighest refractive index and the refractive index does not increase withdistance from the central position.
 47. An optical device according toclaim 46, wherein the refractive index distribution changessubstantially along a quadratic function.
 48. An optical device that iscapable of receiving two signal beams having different wavelengths,multiplexing the signal beams and outputting the signal beams as amultiple signal beam where two different wavelengths are superimposed oneach other, the optical device comprising an optical transmission linebeing sheet-form and including a refractive index distribution such thata highest refractive index part is provided in a direction of athickness of the sheet and a refractive index does not increase withdistance from the highest refractive index part in the direction of thethickness, wherein the two signal beams are made incident on the opticaltransmission line as incident beams, wherein inside the opticaltransmission line, the incident beam is transmitted, in a direction of alength that is orthogonal to the direction of the thickness, in multiplemodes having a plurality of eigenmodes for each wavelength in adirection of a width that is orthogonal to both the direction of thelength and the direction of the thickness, and the exiting beam which isa multiple signal beam is generated in the same position in thedirection of the width according to the wavelength by the plurality ofeigenmodes interfering with each other in the direction of the lengthwith respect to signal beams of the same wavelength, and wherein theexiting beam is made to exit from the optical transmission line.
 49. Anoptical device that connects, by a signal beam, between an externallyinputted input signal and an output signal to be outputted, the opticaldevice comprising: an optical transmission line being sheet-form,including a refractive index distribution such that a highest refractiveindex part is provided in a direction of a thickness of the sheet and arefractive index does not increase with distance from the highestrefractive index part in the direction of the thickness, and comprisinga first partial optical transmission line and a second partial opticaltransmission line adjoining in a direction of the width orthogonal tothe thickness of the thickness; and refractive index modulating meanscapable of changing the refractive index distribution of at least one ofthe first and second partial optical transmission lines based on anexternally supplied control signal, wherein selection can be madebetween a first condition in which the incident beam is transmitted byuse of only the first partial optical transmission line and a secondcondition in which the incident beam is transmitted by use of the firstand second partial optical transmission lines, based on an operation ofthe refractive index modulating means, wherein a signal beamcorresponding to the input signal is made incident on the first opticaltransmission line as the incident beam, wherein in the first condition,inside the first optical transmission line, the incident beam istransmitted, in a direction of a length that is orthogonal to thedirection of the thickness and the direction of the width, in multiplemodes having a plurality of eigenmodes in the direction of the width,the exiting beam is generated by the plurality of eigenmodes interferingwith each other in the direction of the length, and the exiting beam ismade to exit from the first optical transmission line and the outputsignal corresponding to the exiting beam is outputted, and wherein inthe second condition, inside the first and second optical transmissionlines, the incident beam is transmitted, in the direction of thethickness, in multiple modes having a plurality of eigenmodes in thedirection of the width, the exiting beam is generated by the pluralityof eigenmodes interfering with each other in the direction of thelength, and the exiting beam is made to exit from the second opticaltransmission line and the output signal corresponding to the exitingbeam is outputted.
 50. An optical device according to claim 49, whereinthe refractive index modulating means is capable of changing therefractive index distribution of the first multi-mode partial opticaltransmission line, in the second condition, makes the refractive indexdistributions of the first and second multi-mode partial opticaltransmission lines the same as each other, and in the first condition,makes a highest refractive index of the first multi-mode partial opticaltransmission line higher than a highest refractive index of the secondmulti-mode partial optical transmission line.
 51. An optical deviceaccording to claim 49, wherein the refractive index modulating means iscapable of changing the refractive index distribution of the secondmulti-mode partial optical transmission line, in the second condition,makes the refractive index distributions of the first and secondmulti-mode partial optical transmission lines the same as each other,and in the first condition, makes a highest refractive index of thesecond multi-mode partial optical transmission line lower than a highestrefractive index of the first multi-mode optical transmission line. 52.An optical device according to claim 49, wherein the refractive indexmodulating means is capable of changing the refractive indexdistributions of the first and second multi-mode partial opticaltransmission lines, in the second condition, makes the refractive indexdistributions of the first and second multi-mode partial opticaltransmission lines the same as each other, and in the first condition,makes a highest refractive index of the first multi-mode partial opticaltransmission line higher than a highest refractive index of the secondmulti-mode partial optical transmission line in the second condition,and makes the highest refractive index of the second multi-mode partialoptical transmission line lower than the highest refractive index of thefirst multi-mode partial optical transmission line in the secondcondition.
 53. An optical device according to claim 49, wherein of thefirst and second multi-mode optical transmission lines, the opticaltransmission line whose refractive index distribution is changeable bythe refractive index modulating means is made of a polymer exhibiting athermooptic effect, and wherein the refractive index modulating meansincludes a thermal sheet capable of generating/absorbing heat accordingto the control signal, and changes the refractive index distribution bychanging a temperature of the optical transmission line by the thermalsheet.
 54. An optical device according to claim 49, wherein in theoptical transmission line, a size in the direction of the length is avalue that is substantially an odd multiple of the following expressionwhen the basic mode width, in the direction of the width, of thetransmission line is W₀, an effective refractive index of a 0th-ordermode beam excited in the direction of the width is n₀ and the wavelengthof the beam transmitted in the first and second optical transmissionlines is λ: $\frac{4n_{0}W_{0}^{2}}{\lambda}$
 55. An optical deviceaccording to claim 49, wherein the optical transmission line has a size,in the direction of the width, that is (1/{square root}2) times withrespect to the direction of the width to which the optical transmissionline is added.
 56. An optical device according to claim 49, wherein theoptical transmission line includes a refractive index distribution suchthat a central position in the direction of the thickness has thehighest refractive index and the refractive index does not increase withdistance from the central position.
 57. An optical device according toclaim 56, wherein the refractive index distributions changesubstantially along a quadratic function.
 58. An optical device forchanging a distance between a number, N (N=2,3,4, . . . ), of signalbeams disposed on a straight line, wherein a number, N, of opticaltransmission lines are disposed on the straight line, the opticaltransmission lines being sheet-form and including a refractive indexdistribution such that a highest refractive index part is provided in adirection of a thickness of the sheet and a refractive index does notincrease with distance from the highest refractive index part in thedirection of the thickness, wherein the signal beams are made incidenton the optical transmission lines as incident beams, wherein inside theoptical transmission lines, the incident beam is transmitted, in adirection of a length that is orthogonal to the direction of thethickness, in multiple modes having a plurality of eigenmodes in adirection of a width that is orthogonal to both the direction of thelength and the direction of the thickness, and exiting beams aregenerated in positions different from positions where the incident beamsare incident on the optical transmission lines in the direction of thewidth by the plurality of eigenmodes interfering with each other in thedirection of the length, and wherein the exiting beams are made to exitfrom the optical transmission lines as the signal beams.
 59. An opticaldevice according to claim 58, wherein the optical transmission linesinclude: an incident surface for making the incident beams incident; andan exit surface for making the exiting beams exit, and wherein theincident beams are made incident on given positions in the direction ofthe width on the incident surface and the exiting beams are generated inpositions, on the exit surface, whose positions in the direction of thewidth are symmetrical to the incident positions of the incident beamswith respect to a center in the direction of the width.
 60. An opticaldevice according to claim 58, wherein the optical device increases thedistance between the signal beams.
 61. An optical device according toclaim 58, further comprising a sheet-form incident side opticaltransmission line, and the optical transmission line is a 1×N opticalsplitting device that splits one incident beam into a number, N, ofbeams and connects the number, N, of exiting beams into which theincident beam is split, to the optical transmission lines as the signalbeams.
 62. An optical device for changing a position of a signal beam,the optical device comprising a plurality of optical transmission linesbeing sheet-form and including a refractive index distribution such thata highest refractive index part is provided in a direction of athickness of the sheet and a refractive index does not increase withdistance from the highest refractive index part in the direction of thethickness, wherein the plurality of optical transmission lines areconnected in multiple stages so that an exiting beam having exited fromone of the optical transmission lines becomes an incident beam to bemade incident on another one of the optical transmission lines, whereinthe signal beam is made incident on the optical transmission line as theincident beam, wherein inside the optical transmission lines, theincident beam is transmitted, in a direction of a length that isorthogonal to the direction of the thickness, in multiple modes having aplurality of eigenmodes in a direction of a width that is orthogonal toboth the direction of the length and the direction of the thickness, andthe exiting beam is generated in a position different from a positionwhere the incident beam is incident on the optical transmission lines inthe direction of the width by the plurality of eigenmodes interferingwith each other in the direction of the length, and wherein the exitingbeam is made to exit from the optical transmission lines as the signalbeam.
 63. An optical device according to claim 62, wherein the signalbeam is a number, N (N=2,3,4, . . . ), of signal beams disposed on astraight line, wherein a number, N, of optical transmission lines aredisposed on the straight line to change a distance between the number,N, of signal beams, the optical transmission lines being sheet-form andincluding a refractive index distribution such that a highest refractiveindex part is provided in a direction of a thickness of the sheet and arefractive index does not increase with distance from the highestrefractive index part in the direction of the thickness, wherein thesignal beams are made incident on the optical transmission lines as theincident beams, wherein inside the optical transmission lines, theincident beams are transmitted, in the direction of the length that isorthogonal to the direction of the thickness, in multiple modes having aplurality of eigenmodes in the direction of the width that is orthogonalto both the direction of the length and the direction of the thickness,and exiting beams are generated in positions different from positionswhere the incident beams are incident on the optical transmission linesin the direction of the width by the plurality of eigenmodes interferingwith each other in the direction of the length, and wherein the exitingbeams are made to exit from the optical transmission lines as the signalbeam.
 64. An optical device that connects, by a signal beam, between anexternally inputted input signal and an output signal to be outputted,the optical device comprising: a sheet-form optical transmission linebeing sheet-form and including a refractive index distribution such thata highest refractive index part is provided in a direction of athickness of the sheet and a refractive index does not increase withdistance from the highest refractive index part in the direction of thethickness; an incident side optical transmission line that transmits theincident beam corresponding to the input signal so as to be incident onthe sheet-form optical transmission line; an incident side beamconverter that connects the incident side optical transmission line andthe sheet-form optical transmission line and converts a mode field ofthe incident side optical transmission line so that it can be incidenton the sheet-form optical transmission line; an exit side opticaltransmission line that transmits the exiting beam from the sheet-formoptical transmission line so as to exit as the output signal; and anexit side beam converter that connects the exit side opticaltransmission line and the sheet-form optical transmission line andconverts a mode field of the sheet-form optical transmission line sothat it can be incident on the exit side optical transmission line,wherein the signal beam exiting from the incident side beam converter ismade incident on the sheet-form optical transmission line as theincident beam, wherein inside the sheet-form optical transmission line,the incident beam is transmitted, in a direction of a length that isorthogonal to the direction of the thickness, in multiple modes having aplurality of eigenmodes in a direction of a width that is orthogonal toboth the direction of the length and the direction of the thickness, andthe exiting beam is generated by the plurality of eigenmodes interferingwith each other in the direction of the length, and wherein the exitingbeam is made to exit from the sheet-form optical transmission line andmade incident on the exit side beam converter.
 65. An optical deviceaccording to claim 64, wherein the incident side beam converter is alens element having a refractive index distribution such that a highestrefractive index is provided at a center and a refractive indexdecreases with distance from the center, and is disposed in the samenumbers as the signal beams that are made incident on the sheet-formoptical transmission line.
 66. An optical device according to claim 65,wherein the incident side beam converter includes the refractive indexdistribution such that a change in refractive index between the centerand a periphery gradually increases from a side of the incident sideoptical transmission line toward a side of the sheet-form opticaltransmission line.
 67. An optical device according to claim 64, whereinthe incident side beam converter is a slab waveguide having a refractiveindex distribution such that the highest refractive index is provided ina central portion, in a direction parallel to the direction of thethickness, of the sheet-form optical transmission line and therefractive index decreases with distance from the central portion, andis disposed in the same numbers as the signal beams that are madeincident on the sheet-form optical transmission line.
 68. An opticaldevice according to claim 67, wherein the slab waveguide has aconfiguration such that a size in the direction of the width decreasestoward a part of connection with the sheet-form optical transmissionline.
 69. An optical device according to claim 67, wherein the incidentside beam converter is formed integrally with the sheet-form opticaltransmission line.
 70. An optical device according to claim 64, whereinthe incident side beam converter is an optical transmission line havinga refractive index distribution such that a highest refractive index isprovided in a central portion, in a direction parallel to the directionof the thickness and a direction parallel to the direction of the width,of the sheet-form optical transmission line and the refractive indexdecreases with distance from the central portion, and the number ofincident side beam converters disposed for the sheet-form opticaltransmission line is one.
 71. An optical device according to claim 64,wherein the exit side beam converter is a lens element having arefractive index distribution such that a highest refractive index isprovided at a center and a refractive index decreases with distance fromthe center, and is disposed in the same numbers as the signal beamsexiting from the sheet-form optical transmission line.
 72. An opticaldevice according to claim 65, wherein the exit side optical transmissionline is an optical fiber having a refractive index distribution suchthat a highest refractive index is provided at a center and a refractiveindex decreases with distance from the center, and wherein the exit sidebeam converter includes the refractive index distribution such that achange in refractive index between the center and a periphery graduallyincreases from a side of the exit side optical transmission line towarda side of the sheet-form optical transmission line.
 73. An opticaldevice according to claim 64, wherein the exit side beam converter is aslab waveguide having a refractive index distribution such that thehighest refractive index is provided in a central portion, in adirection parallel to the direction of the thickness, of the sheet-formoptical transmission line and the refractive index decreases withdistance from the central portion, and is disposed in the same numbersas the signal beams exiting from the sheet-form optical transmissionline.
 74. An optical device according to claim 73, wherein the slabwaveguide has a configuration such that a size in the direction of thewidth decreases toward a part of connection with the sheet-form opticaltransmission line.
 75. An optical device according to claim 73, whereinthe exit side beam converter is formed integrally with the sheet-formoptical transmission line.
 76. An optical device according to claim 64,wherein the exit side beam converter is an optical transmission linehaving a refractive index distribution such that a highest refractiveindex is provided in a central portion, in a direction parallel to thedirection of the thickness and a direction parallel to the direction ofthe width, of the sheet-form optical transmission line and therefractive index decreases with distance from the central portion, andthe number of exit side beam converters disposed for the sheet-formoptical transmission line is one.
 77. A method of manufacturing anoptical device that connects, by a signal beam, between an externallyinputted input signal and an output signal to be outputted, the opticaldevice comprising: a sheet-form optical transmission line beingsheet-form and including a refractive index distribution such that ahighest refractive index part is provided in a direction of a thicknessof the sheet and a refractive index does not increase with distance fromthe highest refractive index part in the direction of the thickness; anincident side optical transmission line that transmits the incident beamcorresponding to the input signal so as to be incident on the sheet-formoptical transmission line; an incident side beam converter that connectsthe incident side optical transmission line and the sheet-form opticaltransmission line and converts a mode field of the incident side opticaltransmission line so that it can be incident on the sheet-form opticaltransmission line; an exit side optical transmission line that transmitsthe exiting beam from the sheet-form optical transmission line so as toexit as the output signal; and an exit side beam converter that connectsthe exit side optical transmission line and the sheet-form opticaltransmission line and converts a mode field of the sheet-form opticaltransmission line so that it can be incident on the exit side opticaltransmission line, the optical device manufacturing method comprising: afirst step of preparing a forming die that has a concave portioncorresponding to the sheet-form optical transmission line and at leastone of the incident side beam converter and the exit side beam converterand is made of a material capable of transmitting an energy to beapplied to cure a resin of which the sheet-form optical transmissionline is made; a second step of filling the concave portion with theresin; a third step of applying the energy in a predetermined quantityto the forming die filled with the resin, from above and below in thedirection of the thickness to form a desired refractive indexdistribution by curing the resin; and a fourth step of, when theincident side beam converter and the exit side beam converter not formedin the concave portion are present, connecting the converters to thecured resin, and further, connecting the incident side opticaltransmission line and the exit side optical transmission line.
 78. Anoptical device manufacturing method according to claim 77, wherein theapplication of the energy is an application of an ultraviolet ray of apredetermined wavelength, and wherein the forming die is made of amaterial that is transparent with respect to the ultraviolet ray of thepredetermined wavelength.
 79. An optical device manufacturing methodaccording to claims 77, wherein the application of the energy isheating.
 80. An optical device manufacturing method according to claim77, comprising a fifth step of releasing the cured resin from theforming die prior to the fourth step.
 81. An optical devicemanufacturing method according to claim 80, wherein in the fourth step,when the incident side beam converter and the exit side beam converternot formed in the forming die are present, the converters are connectedto the cured resin, and further, when the incident side opticaltransmission line and the exit side optical transmission line areconnected together, the optical transmission lines are disposed on asubstrate where a positioning portion for positioning the opticaltransmission lines is formed.
 82. An optical device manufacturing methodaccording to claim 77, wherein in the first step, the forming dieincludes a positioning portion for positioning at least one of theincident side optical transmission line and the exit side opticaltransmission line, and wherein in the fourth step, the opticaltransmission lines are disposed on the forming die where the positioningportion is formed.
 83. An optical device manufacturing method accordingto claim 77, wherein the incident side optical transmission line is anoptical fiber.
 84. An optical device manufacturing method according toclaim 77, wherein the exit side optical transmission line is an opticalfiber.
 85. An optical device that transmits an externally incidentsignal beam and makes the transmitted signal beam to exit to an outside,the optical device comprising an optical transmission line including arefractive index distribution in a first direction and being capable oftransmitting the signal beam with a plurality of optical paths in asecond direction orthogonal to the first direction, wherein at least oneof an optical axis of the signal beam incident on the opticaltransmission line and an optical axis of the signal beam exiting fromthe optical transmission line is not parallel to the second direction,and wherein a phase difference, at the time of incidence on the opticaltransmission line, between the two optical paths, of the plurality ofoptical paths, incident on the optical transmission line symmetricallyto each other with respect to the optical axis of the signal beam and aphase difference, at the time of exit from the optical transmissionline, between the two optical paths are the same.
 86. An optical deviceaccording to claim 85, comprising: an incident portion for making thesignal beam incident on the optical transmission line; and an exitportion for making the signal beam to exit from the optical transmissionline, wherein at least one of the incident portion and the exit portionis coupled to the optical transmission line so that the optical axis ofthe signal beam transmitted inside is in a direction not parallel to thesecond direction.
 87. An optical device according to claim 86, whereinat least one of the incident portion and the exit portion is coupled tothe optical transmission line so that the optical axis of the signalbeam transmitted inside is orthogonal to the second direction.
 88. Anoptical device according to claim 86, wherein an optical path lengthdifference between the two optical axes is equal to an integral multipleof a wavelength of the transmitted signal beam.
 89. An optical deviceaccording to claim 88, wherein the two optical axes include a number, m(m=1,2,3, . . . ), of optical path length difference generating portionswhere the optical path length difference is caused, and wherein a sum ofthe optical path length differences caused in the number, m, of opticalpath length difference generating portions is equal to a naturalmultiple of the wavelength of the signal beam.
 90. An optical deviceaccording to claim 89, wherein the optical transmission line is asheet-form optical transmission line capable of trapping the signal beamin the first direction, and includes a refractive index distributionsuch that a refractive index in a central portion where a thickness inthe first direction is half is the highest and the refractive index doesnot increase with distance from the central portion in the firstdirection.
 91. An optical device according to claim 90, wherein thesheet-form optical transmission line includes: a first reflectingsurface for bending an optical axis of a signal beam incident from adirection not parallel to the second direction, in the second direction;and a second reflecting surface for bending an optical axis of a signalbeam transmitted in the second direction, in the direction not parallelto the second direction, wherein the optical path length differencegenerating portion is a portion where refractive index histories of thetwo optical paths reflected by the first and second reflecting surfacesare different from each other.
 92. An optical device according to claim90, wherein in the sheet-form optical transmission line, a physicaloptical path length from a position where all of the signal beam is bentin the second direction by the first reflecting surface to a positionimmediately before all of the signal beam is incident on the secondreflecting surface is equal to j (j=0,1,2,3, . . . ) times a period ofmeandering of an optical path transmitted while meandering based on therefractive index distribution.
 93. An optical device according to claim88, wherein the two optical paths include a number, n (n=2,3,4, . . . )of optical path length difference generating portions where an opticalpath length difference is caused, and wherein a sum of the optical pathlength differences caused in the number, n, of optical path lengthdifference generating portions is zero.
 94. An optical device accordingto claim 93, wherein the optical transmission line is a sheet-formoptical transmission line capable of trapping the signal beam in thefirst direction, and includes a refractive index distribution such thata refractive index in a central portion where a thickness in the firstdirection is half is the highest and the refractive index does notincrease with distance from the central portion in the first direction.95. An optical device according to claim 94, wherein the sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, wherein the optical path length differencegenerating portions are portions where refractive index histories of thetwo optical paths reflected by the first and second reflecting surfacesare different from each other.
 96. An optical device according to claim94, wherein in the sheet-form optical transmission line, a physicaloptical path length from a position where all of the signal beam is bentin the second direction by the first reflecting surface to a positionimmediately before all of the signal beam is incident on the secondreflecting surface is equal to (j+0.5) (j=0,1,2,3, . . . ) times aperiod of meandering of an optical path transmitted while meanderingbased on the refractive index distribution.
 97. An optical deviceaccording to claim 86, wherein an optical path length difference betweenthe two optical paths is zero.
 98. An optical device according to claim97, wherein the two optical paths include a number, n (n=2,3,4, . . . )of optical path length difference generating portions where an opticalpath length difference is caused, and wherein a sum of the optical pathlength differences caused in the number, n, of optical path lengthdifference generating portions is zero.
 99. An optical device accordingto claim 98, wherein the optical transmission line is a sheet-formoptical transmission line capable of trapping the signal beam in thefirst direction, and includes a refractive index distribution such thata refractive index in a central portion where a thickness in the firstdirection is half is the highest and the refractive index does notincrease with distance from the central portion in the first direction.100. An optical device according to claim 99, wherein the sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, wherein the optical path length differencegenerating portions are portions where refractive index histories of thetwo optical paths reflected by the first and second reflecting surfacesare different from each other.
 101. An optical device according to claim99, wherein in the sheet-form optical transmission line, a physicaloptical path length from a position where all of the signal beam is bentin the second direction by the first reflecting surface to a positionimmediately before all of the signal beam is incident on the secondreflecting surface is equal to (j+0.5) (j=0,1,2,3, . . . ) times aperiod of meandering of an optical path transmitted while meanderingbased on the refractive index distribution.
 102. An optical deviceaccording to claim 97, wherein the two optical paths do not have aportion where the optical path length difference is caused.
 103. Anoptical device according to claim 102, wherein the optical transmissionline is a sheet-form optical transmission line capable of trapping thesignal beam in the first direction, and includes a refractive indexdistribution such that a refractive index in a central portion where athickness in the first direction is half is the highest and therefractive index does not increase with distance from the centralportion in the first direction.
 104. An optical device according toclaim 103, wherein the sheet-form optical transmission line includes: afirst reflecting surface for bending an optical axis of a signal beamincident from a direction not parallel to the second direction, in thesecond direction; and a second reflecting surface for bending an opticalaxis of a signal beam transmitted in the second direction, in thedirection not parallel to the second direction, wherein a physicaloptical path length between the first reflecting surface and the secondreflecting surface in the central portion is equal to j/2 (j=0,1,2,3, .. . ) times a period of meandering of an optical path transmitted whilemeandering based on the refractive index distribution, and wherein thesignal beam is condensed into a line parallel to a third directionorthogonal to both the first direction and the second direction in thecentral portion where the thickness, in the first direction, of theoptical transmission line is half on the first reflecting surface andthe second reflecting surface.
 105. An optical device that transmits anexternally incident signal beam and makes the transmitted signal beamexit from a predetermined position to an outside by a multi-modeinterference, the optical device comprising: a sheet-form opticaltransmission line including a refractive index distribution in a firstdirection, being capable of transmitting the signal beam in a seconddirection orthogonal to the first direction, and being capable oftrapping the signal beam in the first direction; a number, M (M=1,2,3, .. . ), of incident portions for making the signal beam incident on thesheet-form optical transmission line; and a number, N (N=1,2,3, . . . ),of exit portions for making the signal beam exit from the sheet-formoptical transmission line, wherein the number, M, of incident portionsand the number, N, of exit portions include at least one nonparallelincident and exit portion that is coupled to the sheet-form opticaltransmission line in a direction where an optical axis of the signalbeam transmitted inside is not parallel to the second direction, whereinbetween two optical paths incident on the sheet-form opticaltransmission line symmetrically to each other with respect to theoptical axis of the signal beam, of a plurality of optical paths of thesignal beam transmitted between the nonparallel incident and exitportion and the corresponding incident or exit portion, a phasedifference at the time of incidence on the sheet-form opticaltransmission line and a phase difference at the time of exit from thesheet-form optical transmission line are the same, and wherein thenumber, M, of incident portions and the number, N, of exit portions areall disposed in positions satisfying a predetermined condition of aself-imaging principle of the multi-mode interference.
 106. An opticaldevice according to claim 105, wherein the nonparallel incident and exitportion is coupled to the optical transmission line so that the opticalaxis of the signal beam transmitted inside is orthogonal to the seconddirection.
 107. An optical device according to claim 105, wherein anoptical path length difference between the two optical paths is equal toan integral multiple of a wavelength of the transmitted signal beam.108. An optical device according to claim 107, wherein the two opticalpaths include a number, m (m=1,2,3, . . . ) of optical path lengthdifference generating portions where the optical path length differenceis caused, and wherein a sum of the optical path length differencescaused in the number, m, of optical path length difference generatingportions is equal to a natural multiple of the wavelength of the signalbeam.
 109. An optical device according to claim 108, wherein thesheet-form optical transmission line includes a refractive indexdistribution such that a refractive index in a central portion where athickness in the first direction is half is the highest and therefractive index does not increase with distance from the centralportion in the first direction.
 110. An optical device according toclaim 109, wherein the sheet-form optical transmission line includes: afirst reflecting surface for bending an optical axis of a signal beamincident from a direction not parallel to the second direction, in thesecond direction; and a second reflecting surface for bending an opticalaxis of a signal beam transmitted in the second direction, in thedirection not parallel to the second direction, wherein the optical pathlength difference generating portion is a portion where refractive indexhistories of the two optical paths reflected by the first and secondreflecting surfaces are different from each other.
 111. An opticaldevice according to claim 109, wherein in the sheet-form opticaltransmission line, a physical optical path length from a position whereall of the signal beam is bent in the second direction by the firstreflecting surface to a position immediately before all of the signalbeam is incident on the second reflecting surface is equal to j=0,1,2,3,. . . ) times a period of meandering of an optical path transmittedwhile meandering based on the refractive index distribution.
 112. Anoptical device according to claim 107, wherein the two optical pathsinclude a number, n (n=2,3,4, . . . ) of optical path length differencegenerating portions where the optical path length difference is caused,and wherein a sum of the optical path length differences caused in thenumber, n, of optical path length difference generating portions iszero.
 113. An optical device according to claim 112, wherein thesheet-form optical transmission line includes a refractive indexdistribution such that a refractive index in a central portion where athickness in the first direction is half is the highest and therefractive index does not increase with distance from the centralportion in the first direction.
 114. An optical device according toclaim 113, wherein the sheet-form optical transmission line includes: afirst reflecting surface for bending an optical axis of a signal beamincident from a direction not parallel to the second direction, in thesecond direction; and a second reflecting surface for bending an opticalaxis of a signal beam transmitted in the second direction, in thedirection not parallel to the second direction, wherein the optical pathlength difference generating portions are portions where refractiveindex histories of the two optical paths reflected by the first andsecond reflecting surfaces are different from each other.
 115. Anoptical device according to claim 113, wherein in the sheet-form opticaltransmission line, a physical optical path length from a position whereall of the signal beam is bent in the second direction by the firstreflecting surface to a position immediately before all of the signalbeam is incident on the second reflecting surface is equal to (j+0.5)(j=0,1,2,3, . . . ) times a period of meandering of an optical pathtransmitted while meandering based on the refractive index distribution.116. An optical device according to claim 105, wherein an optical pathlength difference between the two optical paths is zero.
 117. An opticaldevice according to claim 116, wherein the two optical paths include anumber, n (n=2,3,4, . . . ) of optical path length difference generatingportions where the optical path length difference is caused, and whereina sum of the optical path length differences caused in the number, n, ofoptical path length difference generating portions is zero.
 118. Anoptical device according to claim 117, wherein the sheet-form opticaltransmission line includes a refractive index distribution such that arefractive index in a central portion where a thickness in the firstdirection is half is the highest and the refractive index does notincrease with distance from the central portion in the first direction.119. An optical device according to claim 118, wherein the sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, wherein the optical path length differencegenerating portions are portions where refractive index histories of thetwo optical paths reflected by the first and second reflecting surfacesare different from each other.
 120. An optical device according to claim118, wherein in the sheet-form optical transmission line, a physicaloptical path length from a position where all of the signal beam is bentin the second direction by the first reflecting surface to a positionimmediately before all of the signal beam is incident on the secondreflecting surface is equal to (j+0.5) (j=0,1,2,3, . . . ) times aperiod of meandering of an optical path transmitted while meanderingbased on the refractive index distribution.
 121. An optical deviceaccording to claim 116, wherein the two optical paths do not have aportion where the optical path length difference is caused.
 122. Anoptical device according to claim 121, wherein the sheet-form opticaltransmission line includes a refractive index distribution such that arefractive index in a central portion where a thickness in the firstdirection is half is the highest and the refractive index does notincrease with distance from the central portion in the first direction.123. An optical device according to claim 122, wherein the sheet-formoptical transmission line includes: a first reflecting surface forbending an optical axis of a signal beam incident from a direction notparallel to the second direction, in the second direction; and a secondreflecting surface for bending an optical axis of a signal beamtransmitted in the second direction, in the direction not parallel tothe second direction, wherein a physical optical path length between thefirst reflecting surface and the second reflecting surface in thecentral portion is equal to j/2 (j=0,1,2,3, . . . ) times a period ofmeandering of an optical path transmitted while meandering based on therefractive index distribution, and wherein the signal beam is condensedinto a line parallel to a third direction orthogonal to both the firstdirection and the second direction in the central portion where thethickness, in the first direction, of the optical transmission line ishalf on the first reflecting surface and the second reflecting surface.